In a computer game, a builder can build a wall in ten hours while a destroyer can demolish such a wall completely in fourteen hours. Both, the builder and the destroyer were initially set to work together on level ground. But after 7 hours the destroyer was taken out. What was the total time (in hours) taken to build the wall?

Work done by builder in 1 hour is                =  $$\frac{1}{10}$$ unit

Work demolished by destroyer in 1 hour is   = $$\frac{1}{14}$$ unit

Net work done by both of them in 1 hour is  = $$\frac{1}{10}-\frac{1}{14}=\frac{\left(7-5\right)}{70}=\frac{2}{70}=\frac{1}{35}$$ unit

Work done by them in 7 hours                      = $$7\times\frac{1}{35}= \frac{1}5$$ unit

Work left after 7 hours                                 = $$1-\frac{1}{5}=\frac{4}{5}$$ unit

Let time taken by builder to complete remaining work = t hours.

ACT

$$t\times\frac{1}{10} = \frac{4}{5}$$

$$\frac{4}{5}\times\frac{10}{1} = 8 hours$$

Total time taken is $$7+8=15$$ hours.

Hence Option (B) is right answer.