Question

Bj can overhaul a​ boat's diesel inboard engine in 15 hours. his apprentice takes 30 hours to do the same job. how long would it take them working together assuming no gain or loss in​ efficiency?

Answer 1

BJ rate: 1/20 job/hh
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App rate: 1/60 job/hr
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Together rate: 1/x job/hr

Equation:
rate + rate = together rate

1/20 + 1/60 = 1/x

Multiply thru by 60x
3x + x = 60
4x = 60
x = 15 hrs (time to do the job together)