The correct option is D x4 days
x men can do a piece of work in 5 days.
We know that if the number of men increase, then the number of days taken to finish the piece of work will be less.
So, Number of men(p)∝1Number of days(q)
∴p1p2 = q2q1
p1 = x men
p2 = 20 men
q1 = 5 days
and q2=No. of days taken by 20 men
⇒x20 = q25
⇒q2 = x×520
⇒d2 = x4 days