Can you help with my Precalc?

Question

the question says “a rectangular animal pen is built. One side of it is the wall of a barn. It is 1680 ft sq. If Tom can put up the fence in 12 hours, how fast will bob have to work so that the fence is put up in 8 hours? How long will it take bob to do it on his own?”

im so confused. Please help me :(

Answer 1

Tom can do 1/12 fence per hour.

 

Together they must do 1/8 fence per hour.

 

 1/12 + X = 1/8

 

 X = 1/8 - 1/12 = 3/24 - 2/24 = 1/24

 

Bob must be able to construct the fence within a 24 hour period.