Question

Jennifer can clean her room 2 hours, while John can clean the same room in 5 hours. How many hours would it take for them to clean the room together?

Answer 1

the sum of their rates is 1/2 + 1/5
  1 / (1/2 + 1/5) = 10/7
1/2 + 1/5 =1/x
x=10/7
1/2 and 1/5 can interpreted as 1 job per 2 hours and 1 job per 5 hours respectively.
hope this helps

Answer 2

For 1 hour:
- Jennifer can clean 1/2 of her room;
- John cal clean 1/5 of her room.
Both can clean: 1/2 + 1/5 = 5/10 + 2/10 = 7/10
1 hour  -------- 7/10
x hours---------- 1
x = 1 : 7/10 = 10/7 h = 1.42857 h ( or 1 h 25 min 43 s )