Question 885105
 if 2 people work at washing 5 cars and one can do all 5 on their own in 36 minutes and the other can do all 5 on their own in 42 minutes, how long does it take to wash all 5 working together?
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Let t = time required when both together
Let the completed job = 1 (the washing of 5 cars)
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Each will to a fraction of the job, the two fractions add up to 1
{{{t/36}}} + {{{t/42}}} = 1
multiply by the least common multiple of 36 & 42: 252
252*{{{t/36}}} + 252*{{{t/42}}} = 252
Cancel the denominators and you have:
7t + 6t = 252
13t = 252
t = 252/13
t = 19.4 minutes working together
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