An experienced mason is three times as efficient as a new mason and so takes 24 days less to build a wall. In how many days can they build the wall working together?

Explanation

1. 01

   If the experienced mason takes $x$ days, the new one takes $3x$.

2. 02

   $3x - x = 24$, so $x = 12$ and $3x = 36$.

3. 03

   Together: $\frac{1}{12} + \frac{1}{36} = \frac{4}{36} = \frac{1}{9}$, i.e. 9 days.