if it takes 5 men 7 days to dig a hole, how long will it takes 3 men to dig half a hole?

Answer 1

3/5 as many men, so 5/3 as long for one hole.
That is, it will take 3 men 7 * 5/3 = 35/3 days to dig a whole hole.
so how long for 1/2 a hole?

Answer 2

7 * 5 = 35 ... man-days to dig hole

half a hole takes 35/2 man-days

(35/2) / 3 = 35/6 man-days for half a hole ... almost six days