18800 / 470 / 20

Answer: Option C

Explanation:

We need so simple divide 42.5/0.85,
=(4250/85) = 50

Answer: Option C

Explanation:

Remember the formula, if not then cram it :)
\begin{aligned} 2ab = (a^2 + b^2)- (a-b)^2 \end{aligned}
=> 2ab = 29 - 9 = 20
=> ab = 10

Answer: Option B

Explanation:

= (31/10) * (3/10) + (7/5) / 20
= (3.1) * (.3) + (1.4) / 20
= 0.93 + 0.07
= 1

Answer: Option D

Answer: Option D

Explanation:

Let the boys are X, then girls are 7X, total = X+7X = 8X

So it should be multiple of 8, 30 is not a multiple of 8.