A can finish a piece of work in 12 days while B can do it

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A can finish a piece of work in 12 days while B can do it in 15 days. If both work at it together, what time will they take to do the work?

1. 6 days
2. 8 days
3. 6²/₃ days
4. 10¹/₁₅ days

Correct Option: C

Efficiency of A = 100/12 = 8.33%
Efficiency of B = 100/15 = 6.66%
Combined efficiency of A and B = 8.33 + 6.66 = 15%
Number of days taken by A and B, when worked together = 100/15
= 6²/₃ days