A and B together can plough a field in 10 hours but by himself A requires 15 hours. How long would B take to plough the same field?
Select the correct option:
A) 10 hours  B) 20 hours  C) 30 hours  D) 40 hours  E) 50 hours

Rationale: if a and b together can do a piece of work in x days and a alone can do the same work in y days , then b alone can do the same work in x y / y – x days . therefore , the no . of hours required by b = 10 × 15 / 15 – 10 = 150 / 5 = 30 hours . answer : c
Answer: C