Q:

Number of persons * days = work

20 * 7 = 140 man- days

Now, x * 28 = 140 men- days

=> x =5

Therefore in second case the required number of person is 5.

**Second method:**

Since work is constant, therefore M1 x D1= M2 x D2

20 x 7 = M2 x 28

=> M2 = 5