Answer:
49 minutes
Step-by-step explanation:
first we need to find the rate at which a person paints the walls :
it takes 36 minutes to paint 4 walls by 7 people :
so for 7 people the rate at which they paint the walls is :
walls/minute
if we simplify we get 
now that was for 7 people , for 1 person the rate is :
which is 
now we have the rate for one person
so for 9 people they will paint
walls/minute
so it takes them 7 minute to paint 1 wall
which means it takes them 7 x7 = 49 minutes to paint 7 walls