Problema Solution
You can wash one window in 15 minutes and your sister can wash one window in 20 minutes. How many minutes will it take to wash 12 windows if you work together?
Answer provided by our tutors
You can wash one window in 15 minutes, so each minute you can wash 1/15
your sister can wash one window in 20 minutes, so each minute the sister can wash 1/20
If working together, each minute, you two can wash 1/15 + 1/20
The time needed is 1/(1/15+1/20) = 1/(7/60) = 60/7 = 8 4/7