Question 937983
1 pool pump takes 60 minutes to fill up a pool, another pool pump takes 40 minutes to fill up a pool, how long would it take of both pumps were used to fill up the pool? 
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Let t = time (minutes) it takes to fill pool with both pumps
then
t(1/60 + 1/40) = 1
multiply both sides by 120 to get:
t(2 + 3) = 120
t(5) = 120
t = 24 minutes