10 day's work by 15 men = 10/210 = 1/21
At the end of every 10 days 15 additional men are employed i.e., for the next 10 days we have 15 + 15 = 30 men
∴ Next 10 day's work by 30 men = 2/21
Hence in 20 days only (1/21 + 2/21) = 3/21 work is completed
To complete the whole work we have to reach the value of (21/21) work.
Now, (1/21) + (2/21) + (3/21) + ......... (6/21) = 21/21 = 1
Hence total time to complete the whole wok 10 + 10 + 10 + 10 + 10 + 10 = 60 days
