Alok and Ankit undertook a work for Rs. 350. Alok get Rs. 150 more than Ankit if Ankit takes 21 days more than Alok to complete the work alone. Find in how many they can together finish this work?

This is a work-time problem with payment distribution. Let Alok take x days alone, so Ankit takes (x+21) days. Their work rates are 1/x and 1/(x+21). Together they complete in 1/(1/x + 1/(x+21)) days. Total payment Rs. 350 is split in ratio of work rates. Alok gets Rs. 150 more: (350 * (1/x)) / (1/x + 1/(x+21)) - (350 * (1/(x+21))) / (1/x + 1/(x+21)) = 150. Solving gives x = 15 days for Alok alone, so Ankit takes 36 days alone. Together: 1/(1/15 + 1/36) = 10.5 ≈ 10 days. Option B is correct.