# Question Bank - Number Theory Questions From Previous CAT Papers (Solved)

• Let the unit’s place of the original number be y and ten’s place be x.
So, the number is 10x + y.
After reversing, the unit’s place will be x and ten’s place y. Number will be 10y + x.
We need to know, how many numbers when reversed will give an increase of 18.
So, 10y + x – (10x + y) = 18 ; 9 (y – x) = 18 ; y - x = 2
y cannot be 2, as x will be 0 and the number will be 02, which is not a 2 digit number.
y can take values from 3 – 9, and x can take values from 1 – 7 (y = 3, x = 1……y = 4, x = 2 and so on…)
So, there are 6 numbers other than 13 which will satisfy the condition.

• Q7. (CAT 2006)
The number of employees in Obelix Menhir Co. is a prime number and is less than 300. The ratio of the number of employees who are graduates and above, to that of employees who are not, can possibly be:
(1) 101 : 88
(2) 87 : 100
(3) 110 : 111
(4) 85 : 98
(5) 97 : 84

• Solution: The total number of employees will be the sum of graduates and non-graduates and this should be a prime number, and less than 300. To find if the number is prime or not, these are the simple divisibility tests which I use.
2 (if it is even, it is divisible by 2, else not, so ending with 0, 2, 4, 6, 8 divisible by 2)
3 (If sum of digits divisible by 3, then number is divisible by 3)
5 (Number will end with 5 or 0)
7 ( I will rather divide and see)
11 (Add alternate digits, and subtract with the next set of alternate digits. If the difference is 0/multiple of 11, then divisible by 11)
Also, you need to see if the numbers are divisible till their square root (approximately, at least). After their square root, they will not be divisible by any more numbers.
For eg : Consider 70 : Square root of 70 is between 8 & 9. So, we need to check till 8 if all the numbers are divisible or not. (i.e.2, 3, 5, 7). If none of the prime numbers till the square root are not divisible, then the number is a prime number.

Consider the options:
101 : 88, sum is 189. 189 is divisible by 3 (Sum of digits divisible by 3)
87 : 100, sum is 187, 187 is divisible by 11
110 : 111, sum is 221. 221 is divisible by 13. (check prime numbers till 15, as 15^2 is 225 > 223)
85 : 98, sum is 183, divisible by 3.
97 : 84, sum is 181, is a prime number. (Check all prime numbers till 14, as 14^2 is 196 > 181)

Note: Any prime no: > 3 can be written in the form 6k + 1 or 6k - 1. So, out of the given options check if we would be able to write the sum in that format. Option 2 and 5 satisfies and now we can just check only these 2 options for prime. Option 2 is divisible by 11 so answer should be option 5.

• Q8. (CAT 2007)
Consider four digit numbers for which the first two digits are equal and the last two digits are also equal. How many such numbers are perfect squares?
(1) 3
(2) 2
(3) 4
(4) 0
(5) 1

• Solution:

Method1: We know that only for 12^2 last 2 digits are equal.use this property.since number is a 4 digit number the square root has to be a 2 digit number. We know (50-12)^2, (50+120^2, (100-12)^2, (100+12)^2 have same last 2 digits as 12^2.but last one will yield a 5 digit number. Just exclude it and check the rest 3. You'll get only (100-12)^2 = 88^2 satisfying the criteria. so option 5

Method 2: Let the number be aabb.
aabb = 1000a + 100a + 10b + b = 1100a + 110 b.
1100 and 110 are divisible by 11 and 11 is a prime number. i.e. 1100a + 11b = 11 (100a + b)
Since aabb sould be a perfect square, and 11 is already a factor of that, there should be “one more” 11 that should be a factor. (so that it becomes a perfect square)
So, 121 should be a factor of aabb.
The number can be 121 * x, where x should be another perfect square.
The numbers could be : 121 * 9, 121 * 16, 121 * 25, 121 * 36, 121 * 49, 121 * 64, 121 * 81
Out of this, 121 * 64 is 7744, which is of the form aabb. None of the others are.

• Q9. (CAT 2001)
All the page numbers from a book are added, beginning at page 1. However, one page number was mistakenly added twice. The sum obtained was 1000. Which page number was added twice?
(1) 44
(2) 45
(3) 10
(4) 12

• Solution: Sum of n natural numbers = n (n+1)/2
here, n( n + 1)/2 = 1000 - x (where x is the number that is added twice)
n(n+1) < 2000 - 2x
we need to find max value n can take . We know for a product of two numbers to be max the number should be closest... find out the least number whose square is higher than 2000.
45^2 = 2025

Will break here to learn a neat trick :)
45^2 = [ 4 (digit(s) excluding 5) * (4 (digit(s) excluding 5) + 1)]  = 2025.
Similarly 135^2 = [13 x 14]  = 18225. Got it ? Good, back to our problem!

So our number is somewhere near to 45.. it cant be more than 45 as our required product is less than 2000.
44 * 45 = 1980.. Cool! so we can have n = 44
n (n + 1) = 44 * 45 = 1980 = 2000 - 2 * 10
x = 10 ( so we have 44 pages and page number 10 was added twice) Option 3

• Q10. (CAT 2001)
For a Fibonacci sequence, from the third term onwards, each term in the sequence is the sum of the previous two terms in that sequence. If the difference in squares of seventh and sixth terms of this sequence is 517, what is the tenth term of this sequence?
(1) 147
(2) 76
(3) 123
(4) Cannot be determined

• f(7)^2 - f(6)^2 = 517
(f(7) + f(6)) (f(7) - f(6)) = 517 = 11 x 47
As 11 and 47 cannot be further factorized and f(7) or f(6) should be natural numbers, we can say
f(7) + f(6) = 47 (Largest prime factor should come from their sum)
f(7) - f(6) = 11 (smallest prime factor should come from their difference)
solving, f(7) = 29 and f(6) = 18

f(10) = f(8) + f(9)
= f(7) + f(6) + f(7) + f(8)
= f(7) + f(6) + f(7) + f(7) + f(6)
3f(7) + 2f(6) = 3 x 29 + 2 x 18 = 123.

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