Gopal can whitewash a school building in 16 days. Two helpers, Hema and Iqbal, could do it alone in 24 and 48 days. Gopal works every day, but the helpers join him only on every third day (day 3, day 6, day 9, …). In how many days is the building whitewashed?

Explanation

1. 01

   In each 3-day block Gopal does $3 \times \frac{1}{16} = \frac{9}{48}$; the helpers add $\frac{1}{24} + \frac{1}{48} = \frac{3}{48}$.

2. 02

   One block = $\frac{12}{48} = \frac{1}{4}$ of the building.

3. 03

   4 blocks finish it, and the 4th block ends on a helper day, so the time is $4 \times 3 = 12$ days.