The average of 20 numbers is zero. Of them, at the most, how many may be greater than zero?
Options:
(A) 17
(B) 16
(C) 17
(D) 19
(E) 62

Average of 20 numbers = 0
Sum of 20 numbers = (0 * 20) = 0
It is quite possible that 19 of these numbers may be positive and if their sum is a, then 20th number is (-a). So the answer is: (D).

A can do a piece of work in 10 days. He works at it for 4 days and then B finishes it in 9 days. In how many days can A and B together finish the work?
Options:
(A) 6
(B) 8
(C) 12
(D) 15
(E) 20

4/10+9/X = 1/x = 15
1/10+1/15 = 1/6 = 6 days So the answer is: (A).

If 12x = 16y = 28z, then what is a possible sum of positive integers x, y, and z?
Options:
(A) 52
(B) 58
(C) 122
(D) 84
(E) 168

12x = 16y = 28z
3x = 4y = 7z
3(4*7) = 4(3*7) = 7(3*4)
Addition = 28+21+12 = 61
Answer would be multiple of 61 which is 122 So the answer is: (C).

Find the invalid no.from the following series 13, 18, 25, 30, 37, 40
Options:
(A) 13
(B) 18
(C) 40
(D) 37
(E) 40
The differences between two successive terms from the beginning are 7, 5, 7, 5, 7, 5. So, 40 is wrong. So the answer is: (C).