1/30 + 1/40 = 7/120
120/7 = 17 1/7 days
Ravi can do a piece of work in 30 days while Prakash can do it in 40 days. In how many days will they finish it together?
a: 17 1/7
b: 17 1/9
c: 17 1/2
d: 17 1/7
e: 17 1/5