A certain number of men can do a piece of work in 80 days. If there were 10 men less, it could be finished in 20 days more. How many men are there in the starting?

Let the original number of men = N
Time taken by N = 80 days
Now, (N - 10) men can finish the work in the (80 + 20) = 100 days
Here, M₁ = N, M₂ = (N - 10), D₁ = 80 and D₂ = 100
According to the formula.
M₁D₁ = M₂D₂
⇒ N x 80 = 100 x (N - 10)
⇒ 8N = 10N - 100
⇒ 10N - 8N = 100
⇒ N = 50