Question 692384
One pipe can be used to fill a tank in 48 min and another can be used to fill it in 1 hour and 12 min.
 How long will it take to fill the tank if both pipes are used. 
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Convert 1 hr 12 min, to min: 60 + 12 = 72 min
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Let t = time (in min) to fill the tank using both pipes
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Let the competed job = 1; (a full tank)
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Each pipe will do a fraction of the job, the two fractions add up to 1
:
{{{t/48}}} + {{{t/72}}} = 1
Multiply by the least common multiple to clear the denominators
144*{{{t/48}}} + 144*{{{t/72}}} = 144
cancel the denominators and you have
3t + 2t = 144
5t = 144
t = 144/5
t = 28.8 min working together
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Check this using a calc
{{{28.8/48}}} + {{{28.8/72}}} =
.6 + .4 = 1; confirms our solution to 28.8