mathalgebra
posted by amanda .
one pump can empty a pool in 5 days, wheras a second pump can emtpy pool in 7 days. how long will it take the two pumps, working together, to empty the pool?

mathalgebra 
Reiny
rate of 1st pump = pool/5
rate of 2nd pump = pool/7
combined rate =pool/5 + pool/7 = 12 pool/35
time at combined time = pool/(12pool/35)
= 35/12 = 2.91666.. days
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