Question 896788
Wow! You guys are fast!

60/3=20
1/20
75/5=15

1/20+1/15=7/x
3/60+4/60=7/x
7/60=7/x
1/60=1/x
x=60 minutes or one hour to paint 7 rooms working together

check
60/20+60/15=3+4=7

ok