If it takes painter # 1 three hours to paint a house, and painter # 2 eight hours to paint a house, how long would it take them if they work together?

Answer 1

#1's rate = house/3
#2's rate = house/8
combined rate = house/3 + house/8
= 11house/24

so time for combined effort = house/(11house/24)
= 24/11 hours