Problema Solution
There are 720 dishes that need to be rinsed. John can rinse them in 72 minutes by himself. It will take his friend Bob 144 minutes to rinse these dishes. How long will it take them if they rinse these 720 dishes together.
Answer provided by our tutors
the rate of John is 720 dishes in 72 min or 720/72 = 10 dishes per min
the rate of Bob is 720 dishes in 144 min or 720/144 = 5 dishes per min
let 't' be the time the need to rinse the 720 dishes together then
t*10 + t*5 = 720
by solving we find
t = 48 min
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it will take them 48 minutes to rinse 720 dishes together.