A father is now three times as old as his son. Five years back, he was four times as old as his son. The age of the son is :

Answer: Option D

Explanation:

If we read given question carefully, we get,
B+8 = C ---(i)
A-8 = C-3 ---(ii)
A+6 = 2D ---(iii)
B+D = 50 ---(iv)

Putting C = A -5 from (ii) into (i), we have :
B+8 = A-5 or A-B = 13 ---(v)

Putting D = 50-B from (iv) and (iii), we have :

A+6 = 100-2B or A+2B = 94 ---(vi)

Solving (v) and (vi), we get
B = 27 and A = 40

Answer: Option C

Explanation:

Let number of girls = x and number of boys = 3x.

Then, 3x + x = 4x = total number of students.

Thus, to find exact value of x, the total number of students must be divisible by 4.

Answer: Option D

Explanation:

Let the number of keepers be x.
Then, total number of feet = 2*50+4*45+4*8+2x
= 2x + 312

Total number of heads = 50+45+8+x = 103 + x
Now, (2x+312) = (103+x)+224
=> x = 15

Answer: Option B

Explanation:

Let the number of boys be x.
Then, 3/4 x = 18
=> x = 24

If total number of students is y, then
2/3 y = 24
=> y = 36

Number of girls in class = y - x
= 36 - 24 = 12

Answer: Option B