A and B together can complete a piece of work in 12 days and B and C together in 15 days. If A is twice as good a workman as C, then in how many days will B alone complete the same work? A और B एक साथ किसी काम को 12 दिन में कर सकते है तथा B और C एक साथ 15 दिन में कर सकते है। यदि A, C से दोगुना कुशल कारीगर है तो B अकेला इसी काम को कितने दिनों में पूरा करेगा?

Let A, B, C have work rates of a, b, c respectively (work per day). From given: a + b = 1/12, b + c = 1/15. Since A is twice as efficient as C, a = 2c. Solving: (a + b) - (b + c) = 1/12 - 1/15, so a - c = 1/60. Substituting a = 2c: 2c - c = 1/60, so c = 1/60. Then b = 1/15 - 1/60 = 4/60 - 1/60 = 3/60 = 1/20. Therefore B alone takes 20 days. Verify: A's rate = 2/60 = 1/30, so A + B = 1/30 + 1/20 = 5/60 = 1/12 ✓, and B + C = 1/20 + 1/60 = 4/60 = 1/15 ✓.