Question

John can do a piece of work in 9 days. But with the help of his classmate, he can do the same work in 3 days. In how many days can his classmate does the work?​

Answer 1

$1 \: hour \: work \: of \: John = \frac{1}{9}$

$1 \: hour \: work \: of \: Classmate = \frac{1}{3}$

$1 \: hour \: of \: both = \frac{1}{9} + \frac{1}{3}$

$= \frac{1 + 3}{9}$

$= \frac{4}{9}$

$So \: Together \: can \: do \: this \: job$

$= \frac{9}{4} \times 100$

$= 25 \times 9$

$= 225 \: minutes$