Question

Tommy can paint a fence in 9 hours. If his friend Huck helps they can paint it in 1 hour. How fast could Huck paint the fence by himself?

Answer 1

Tommy can paint 1/9 of a fence in 1 hour.
With Huck he can paint a whole fence in 1 hour. 
x : part of a fence that Huck can paint alone in 1 hour.
1/9 + x = 1
x = 1 - 1/9 = 8/9
1 : 8/9 = 9/8 = 1.125 hours = 1 hour 7 minutes 30 seconds