X men can finish a piece of work in 30 days. If there were 6 men more, the work could be finished in 10 days less. The original number of men is:

6 10 12 15

12

(Total work = No. of men × no. of days) ATQ, X × 30 =  (X+6) × 20 X × 30 =  (X × 20) + (6 × 20) 30X - 20X = 120 10X = 120 X = 12