Question

it takes tom 4 hours to build a fence. If he hires jack to help him, together they can do the job in just 3 hours. If jack built the same fence alone, how long would it take him?

Answer 1

4 hours because together is faster alone is harder

Answer 2

 Tom:
time to build a fence= 4 hours; rate =1/4 job/hr.

tom &jack 
time to build a fence = 3 hrs.; rate = 1/3 job/hr

Jack :
time to build a fence= x hrs ; rate = 1/x job/hr


1/4 + 1/x = 1/3
3x + 12 = 4x
x=12 hrs. (Time for jack to build a fence )