Sneha can plaster a room in 6 days and Irfan in 9 days. They work on alternate days, with Sneha working on the first day. In how many days is the room plastered?

Explanation

1. 01

   Two-day cycle: $\frac{1}{6} + \frac{1}{9} = \frac{3+2}{18} = \frac{5}{18}$.

2. 02

   3 cycles (6 days) finish $\frac{15}{18}$, leaving $\frac{3}{18} = \frac{1}{6}$.

3. 03

   Day 7 is Sneha's, and she does exactly $\frac{1}{6}$ in a day.

4. 04

   So the room is done in 7 days.