Question 257730
 The swimming pool can be filled in 12 hours with a pipe or 30 hours using a hose. How long will it take if both are used?"
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Let t = time required using both
:
Let the completed job = 1 (a full pool)
:
Each device will do a fraction of the job, the two fractions add up to 1
{{{t/12}}} + {{{t/30}}} = 1
Multiply by a common multiple to get rid of the denominators, 60 will work, results
5t + 2t = 60
7t = 60
t = {{{60/7}}}
t = 8.57 hrs working together
:
:
See if that's true, check on calc:
8.57/12 + 8.57/30 = 
.714 + .286 = 1