Question : A and B together can finish a work in 7.2 hours. If B alone can finish the same work in 18 hours, then A alone can do it in:
Option 1: 18 hours
Option 2: 12 hours
Option 3: 10 hours
Option 4: 15 hours
Correct Answer: 12 hours
Solution :
Hourly work of A and B = $\frac{1}{7.2}$
Hourly work of B = $\frac{1}{18}$
Hourly work of A = $\frac{1}{7.2}-\frac{1}{18}$
= $\frac{10-4}{72}$
= $\frac{6}{72}$
$\therefore$ A alone can complete the work in $\frac{72}{6}=12$ hours
Hence, the correct answer is 12 hours.
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