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### Question

A and B can together complete a job in 21 days. A can alone complete it in 28 days. How long would B alone take to finish the job?

**A**

84 days.

**B**

85 days.

**C**

83 days.

**D**

87 days.

**Soln.**

**Ans: a**

One day work of A + B is $1/21$. One day work of A is $1/28$. So one day work of B, say, $1/y$ = $1/21$ - $1/28$. Solving, we get y = 84 days.