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. (10 points)

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

Step-by-step explanation: