Aptitude - Numbers

Exercise : Numbers - General Questions
56.
 9 + 3 + 7 + 2 - 9 + 1 = ? 4 17 15
 7 + 719 1020
 9 + 817 1020
 9 + 719 1020
 7 + 817 1020
None of these
Explanation:
Given sum
 = 9 + 3 + 7 + 2 - 9 + 1 4 17 15
 = (9 + 7 - 9) + 3 + 2 - 1 4 17 15
 = 7 + 765 + 120 - 68 1020
 = 7 + 817 1020

57.
 1 - 1 + 1 - 2 + 1 - 3 + ... up to n terms = ? n n n
 1 n 2
 1 (n - 1) 2
 1 n(n - 1) 2
None of these
Explanation:
Given sum
 = (1 + 1 + 1 + ... to n terms) - 1 + 2 + 3 + ... to n terms n n n
 = n - n 1 + 1 [ Ref: nth terms = (n/n) = 1] 2 n
 = n - n + 1 2
 = 1 (n - 1) 2

58.
On dividing 2272 as well as 875 by 3-digit number N, we get the same remainder. The sum of the digits of N is:
10
11
12
13
Explanation:

Clearly, (2272 - 875) = 1397, is exactly divisible by N.

Now, 1397 = 11 x 127

The required 3-digit number is 127, the sum of whose digits is 10.

59.
A boy multiplied 987 by a certain number and obtained 559981 as his answer. If in the answer both 9 are wrong and the other digits are correct, then the correct answer would be:
553681
555181
555681
556581
Explanation:

987 = 3 x 7 x 47

So, the required number must be divisible by each one of 3, 7, 47

553681 (Sum of digits = 28, not divisible by 3)

555181 (Sum of digits = 25, not divisible by 3)

555681 is divisible by 3, 7, 47.

60.
How many prime numbers are less than 50 ?
16
15
14
18