Question

A man do a piece of work in $40$ days. He worked at it for $5$ days, then B finished it in $21$ days. The number of days that A and B will take together to finish the same work is

• A. $12days$
• B. $10days$
• C. $15days$
• D. $25days$

Answer 1

Answer: (C)

$15days$

Work done by A and B in $8min$.
$=8\times (\frac{1}{24}+\frac{1}{30})$
$=8\times \left(\frac{5+4}{120}\right)=\frac{9}{15}=\frac{3}{5}$.
Remaining work $1-\frac{3}{5}=\frac{2}{5}th$
$\frac{2}{5}$ of the tank is filled by B in $\frac{2}{5}\times 30=12min$.