1/45+ 1/30 = 5/90 = 1/18
18/1 = 18*4 = 72 days
A can do a job in 45 days and B can do it in 30 days. A and B working together will finish four times the amount of work in ------- days?

[A] 30days
[B] 45 days
[C] 80 days
[D] 72 1/2 days
[E] 72 days