A and B can do a work in 4 hours and 12 hours respectively. A starts the work at 6AM and they work alternately for one hour each. When will the work be completed? Choices.
A) 3
B) 4
C) 5
D) 6
E) 78

Rationale: "work done by a and b in the first two hours , working alternately = first hour a + second hour b = 1 / 4 + 1 / 12 = 1 / 3 . total time required to complete the work = 2 * 3 = 6 days . answer : d"
Answer: D