Two college students have started their own business buildin

Two college students have started their own business building computers from kits. Working alone, one student can build a computer in 20 hours. When the second student helps, they can build a computer in 7.5 hours. How long would it take the second student, working alone to build the computer?

Solution

If second student can build the house in x hrs

Then in 1hr he can build 1/x of the building

First student can build in 1 hr --- 1/20 th of the building

Together they can build the building in 1/ ( 1/x +1/20) =7.5

1/ ( 1/x +1/20) =7.5

1/7.5 = (20+x)/20x

20x = 7.5(20+x)

12.5x = 7.5*20

x = 12hrs .the other student can alone build it in 12 hrs