20 men can dig a tunnel in 30 days. If after 10 days, 20 more men are deployed to dig the tunnel, how many more days it will take to dig the whole tunnel? Choose one
A) 10 B) 15 C) 20 D) 25 E) 30

Rationale: a remaining work can be completed by 20 men in 30 - 10 = 20 days . if 20 men can do x work in 20 days , and let us say 40 men take n days to do the same work . now , more men , less number of days ( inversely proportion ) 20 : 40 : : n : 20 n = 20 x 20 / 40 = 10 days .
Answer: A