Question

Mark and clean his father's office in 30 minutes his younger sister Lynn can clean the office in 40 minutes how long will it take the two of them together to clean the office

Answer 1

we are given

Mark clean his father's office in 30 minutes

so, the amount of work done by Mark in 1 minute is $\frac{1}{30}$

younger sister Lynn can clean the office in 40 minutes

so, the amount of work done by younger sister Lynn in 1 minute is $\frac{1}{40}$

total work done by them in 1 minute is $\frac{1}{30}+\frac{1}{40}$

now, we can simplify it

$=\frac{4*1}{4*30}+\frac{3*1}{3*40}$

$=\frac{4+3}{120}$

$=\frac{7}{120}$

total time taken to complete same work is $\frac{120}{7} min$........Answer

Answer 2

Id guess about 17.5 minutes. 30/2=15....40/2=20   20+15=35... 35/2=17.5