Question

A boy spend 3 by 4 of his pocket money and then 4 by 5 of the remainder is given to his sister if he has ₹40 left what did he have at first

Answer 1

The boy has (1-(3/4))*(1-(4/5)) times his pocket money left, so, he has (1/4)*(1/5) of his pocket money, considered x. Then he originally had 20 times Rs. 40 that is Rs. 800

Answer 2

Let the boy had Rs. X at the beginning.

He spent $\frac{3}{4}$ of X.
⇒ He's left with X - $\frac{3}{4}$ of X. = $\frac{1}{4}$X

He gives $\frac{4}{5}$ of $\frac{1}{4}$ of X = $\frac{1}{5}$ X to sister.
⇒He's left with $\frac{1}{4}$X -  $\frac{1}{5}$X =  $\frac{1}{20}$X.

Given 
 $\frac{1}{20}$X = Rs. 40

⇒ X = Rs. 800

Therefore the boy had ₹800 in the beginning.