It takes 36 minutes for 7 people to paint 4 walls. how many minutes does it take 9 people to paint 7 walls?

Seeing as 7 people get 4 walls in 36 minutes, we can say that the amount of walls one person gets done in 36 minutes is 4/7 (due to that if 7 people=4 walls, we can divide both sides by 7 to get 4/7 walls = 1 person). and the amount that one person gets done in one minute is 4/(7*36)=4/252=1/63 due to that, assuming that everyone works at the same rate, they do 1/36th of the work in 1/36th of the time. Since we need to find how long it takes for 9 people to paint 7 walls, we can start by getting the amount of walls done per minute for 9 people - since 1 person is 1/63, we can multiply that by 9 to get 9/63=1/7. Next, we have to figure out how many minutes (1/7ths of walls) it will take to get to 7 walls - to find that, we can divide 7 by 1/7 to get 
(7/1)/(1/7)=(7*7)/1=49