Ashank's SHORTCUT TECHNIQUES EBOOKLET PDF
Ashank's SHORTCUT TECHNIQUES EBOOKLET PDF
Ashank's SHORTCUT TECHNIQUES EBOOKLET PDF
Are You
Preparing For
CAT 2018 ?
Problem Solving Techniques Booklet
3) Numbers ending in 3, 7, 9 Change the power so that the base e.g.) 17 =(17 ) (taking power 4 as 7
ends with 1 and then use the same will end in 1.
2 2 72
technique as for those numbers (17 x17 )
ending with 1.
4 4 2
eg) 3 , 7 &9 all will end in1.
72 2
=( _89*_89) (as last 2 digits of 17 =89)
72
= ( _21) (as last 2 digits of 89*89=21)
Answer =__41( as 2*2=4)
4) For even numbers (2,4,6,8) Use the pattern of the number 1024
10 788 10 78 8
=2 i.e. e.g.) 2 = (2 ) x 2 =
10
x 2 raised to even power ends with
76 and
10
x 2 raised to odd power ends with
24.
__76 x __56 = __56.
It is also important to note that,
" 76 multiplied by the last 2 digits of any power of 2 will end in the same last 2 digits
E.g. 76 x 04 = 04, 76 x 08 = 08, 76 x 16 = 16, 76 x 32 = 32
2 2 2 2 2 2 2 2 2
" The last two digits of, x , (50-x) , (50+x) , (100-x) will always be the same. For example last 2 digits of 12 , 38 , 62 , 88 , 112 ….
will all be the same.
2 2 2 2 2 2 2 2
Also, last two digits of 11 =39 =61 =89 =111 =139 =161 =189 and so on
3. To find the squares of numbers from 30-70 we can use the following method
2
To find 41
Step1: Difference from 25 will be first 2 digits = 16
Step 2: Square of the difference from 50 will be last 2 digits = 81. Answer = 1681.
2
To find 43
Step1: Difference from 25 will be first 2 digits = 18
Step 2: Square of the difference from 50 will be last 2 digits = 49. Answer = 1849
Combining all these techniques we can find the last 2 digits for any number because every even number can be written as 2* an
odd number
4) “MINIMUM OF ALL” REGIONS IN VENN DIAGRAMS
1. In a survey conducted among 100 men in a company, 100 men use brand A, 75 use brand B, 80 use brand C, 90 use brand D &
60 use brand E of the same product. What is the minimum possible number of men using all the 5 brands, if all the 100 men
use at least one of these brands?
Sum of the difference from 100 = (100-100) + (100-75) + (100-80) + (100-90) + (100-60) = 95
Again take the difference from 100 = 5 (answer)
This method will be explained in detail in the next booklet.
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CAT
5) SIMILAR TO DIFFERENT GROUPING IN PERMUTATION & COMBINATION
All questions in Permutation and Combination fall into 4 categories, and if you master these 4 categories, you can understand all
concepts in P&C easily.
(i) Similar to Different
(ii) Different to Similar
(iii) Similar to Similar
(iv) Different to Different
In this booklet, we will look at the first category; i.e. Similar to Different, which entails the number of ways of dividing ‘n’
identical (similar) things into ‘r’ distinct (different) groups
(a) NO LIMIT QUESTIONS
Let me explain this with an example. Suppose I have 10 identical chocolates to be divided among 3 people. The 10 chocolates
need to be distributed into 3 parts where a part can have zero or more chocolates.
So let us represent chocolates by blue balls. The straight red lines are used to divide them into parts. So you can see that for
dividing into 3 parts, you need only two lines.
st nd rd
Suppose you want to give 1 person 1 chocolate, 2 3 chocolates and 3 6 chocolates. Then you can show it as:
Suppose you want to give one person 1 chocolate, another person 6 chocolates and another one 3, then it can be represented
as:
Now if first person gets 0, second gets 1 and third gets 9 chocolates then it can be represented as:
Now suppose you want to give first person 0, second also 0 and third all of 10 then you can show it like:
So, for dividing 10 identical chocolates among 3 persons you can assume to have 12 (10 balls +2 sticks) things among which ten
are identical and rest 2 are same and of one kind. So number of ways in which you can distribute ten chocolates among 3
people is the same in which you can arrange 12 things among which 10 are identical and of one kind while 2 are identical and of
one kind which can be done in 12!/(10!2!)
The above situation is same as finding the number of positive integral solutions of a + b + c = 10. a, b, c is the number of
chocolates given to different persons.
st
Or else I can also say how many integral points lie on the line a + b + c = 10 in the 1 quadrant. In both the cases the answer is
12
C2.
(VI) Right most non zero integer in a factorial
7. What is the last non – zero digit of 20!
Solution: Using above property we can write:
1 x 2 x3 x 4 x5 x6 x7 x8 x9 x10 = 4 x
11 x 12 x13 x 14 x15 x16 x17 x18 x19 x20 = 4 x
So last non – zero digit of 20! = x last non – zero digit of 4! = last non – zero digit of 6 x 4 = 4
APPLICATION QUESTION BASED ON FACTORIAL
8. How many natural numbers are there such that their factorials are ending with 5 zeroes?
10! is 1x2x3x4x(5)x6x7x8x9x(2x5). From this we can see that highest power of 5 till 10! is 2. Continuing like this, 10!-14!, highest
power of 5 will be 2. The next 5 will be obtained at 15 = (5*3).
Therefore, from 15! To 19! - The highest power of 5 will be 3.
20!-24! – Highest Power = 4, In 25, we are getting one extra five, as 25 = 5x5. Therefore, 25! to 29!, we will get highest power of
5 as 6. The answer to the question is therefore, 0. There are no natural numbers whose factorials end with 5 zeroes.
7) USE OF GRAPHICAL DIVISION IN GEOMETRY
Let’s look at a technique which will help you solve a geometry question in no time
1. ABCD is a square and E and F are the midpoints of AB and BC respectively. Find the ratio of Area (ABCD) : Area (DEF)?
Let’s divide the figure using dotted lines as shown in Figure B. Area of ABCD=100%. Area AEDG=50%. Then Area in shaded region
1(AED) = 25%. Similarly, DCFH=50%. Area in shaded region 2(DCF)=25%. Now Area of EOFB= 25%. .Area of shaded region 3(BEF)
=12.5%. Total area outside triangle= 62.5%. Area inside triangle= 100-62.5=37.5%. Required ratio = 100/37.5 = 8:3
8) ASSUMPTION method
There are at least 10 questions out of 25 in CAT 2007 where you can apply this technique. This involves assuming simple values
for the variables in the questions, and substituting in answer options based on those values. Assumption helps to tremendously
speedup the process of evaluating the answer as shown below.
2 2 2
1. k & 2k are the two roots of the equation x – px + q. Find q + 4q + 6pq =
2 3 3
(a) q (b) p (c) 0 (d) 2p
Solution: Assume an equation with roots 1&2 (k=1)
2
=>p (sum of roots)= 3 and q(product of roots)=2. Substitute in q + 4q + 6pq = 54. Look in the answer options for 54 on
3 3
substituting values of p=3 and q=2. we get 2p = 54. => Answer = 2p .
3 3
2. Let ‘x’ be the arithmetic mean and y,z be the two geometric means between any two positive numbers. The value of (y + z ) /
(xyz) is
(a) 2 (b) 3 (c) 1/2 (d) 3/2
Solution: Assume a GP 1 1 1 1 then x = 1, y = 1 z = 1. Answer on substitution=2, which will make the calculation even faster,
half of the problems in Algebra can be solved using assumption. This is not direct substitution. In the next e.g.: see how you can
use the same technique in an equation question.
#SOME ACTUAL QUESTIONS FROM CAT SOLVED USING THIS TECHNIQUE
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CAT
1. Consider the set S={2,3,4……2n+1), where n is a positive integer larger than 2007. Define X as the average of odd integers in S
and Y as the average of the even integers in S. What is the value of X-Y?
(a) 1 (b) n/2 (c) (n+1)/2n (d) 2008 (e) 0
Solution: The question is independent of n, which is shown below.
Take n=2. Then S= {2,3,4,5). X= 4 and Y=3. X-Y =1, Take n=3. then S={2,3,4,5,6,7}. X=5 and Y=4. X-Y=1
Hence you can directly mark the answer option (a) .You can solve the question in less than 60 seconds.
2. Let S be the set of all pairs (i, j) where 1≤i<j≤n and n≥4. Any two distinct members of S are called friends, if they have one
constituent of the pairs in common and “enemies” otherwise. For example, if n=4, then S={(1,2),(1,3),(1,4),(2,3),(2,4),(3,4)}.
Here (1,2), (1,3) are friends but (1,4), (2,3) are enemies. For general n, consider any two members of S that are friends. How
many other members of S will be common friends of these members?
2 2
(a) 2n-6 (b) n/2(n-3) (c) n-2 (d) (n -7n+16)/2 e) (n -5n+8)/2
Solution: We will start with n=5, then you will get two options (c) and (d).
Now take n =6, lets take two members (1,2) and (1,3). Then their common friends will be ( 1,4), (1,5),(1,6) and (2,3).i.e Four
common friends. So substitute n=6 among answer options and check where you get answer 4.Only option (c).
There were more questions which could be solved using similar strategies. The methods given above clearly show that for
someone with good conceptual knowledge and right strategies the quant section is a cakewalk.
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About the trainers of Ashank Dubey
" Ashank is unarguably India's best Quant trainer and 100%iler of CAT – 16 (Overall) & CAT – 15 (Qunat). He has
trained numerous CAT toppers ever since the CAT went online, including 99.9%tilers of CAT 2015 – Shrikanth,
Rushil, Sandeep, Abhilasha, Prakhar, Pratik, Shivani to name a few.
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