Given in the question,
Let y be the persons there originally .
Work done by y persons can finish = 100 days
If there were 10 persons less, it would take more days finish the work in 10 days
i.e. ( y - 10 ) = 110
According to question ,
y × 100 = ( y - 10 ) × 110
⇒ 100y = 110y - 1100 ⇒ 10y = 1100 ⇒ y = 110
Therefore , 110 persons were there originally .
