Question

It takes Brian 15 hours longer to build a model car than it takes John. If they work together, they can build the model car in 4 hours. Using complete sentences, explain each step in figuring out how to determine the time it would take Brian to build the car on his own.

Answer 1

When they work together, it takes John 1/x hours and it takes Brian 1/(x+15) hours to complete 1/4 of the model.
1/x + 1/( x+15 ) = 1/4 / · x ( x+15 )
x+15 + x = x ( x+15 ) /4 / · 4
8 x + 60 = x² + 15 x
x² + 15 x - 8 x - 60 = 0
x² + 7 x - 60 = 0
x² + 12 x - 5 x - 60 = 0
x · ( x + 12 ) - 5 ·( x+ 12 ) = 0
( x + 12 ) · ( x - 5 ) =0
x = - 12 ( not accepted ) or x = 5
The time taken by Brian: 5 + 15 = 20 hours