Answer:
16 minutes
Step-by-step explanation:
If it takes 63 minutes to paint 9 walls then it means for every wall, it takes 63/9=7 minutes
Since it is 4 people working for 7 mins, the individual rate is 4/7 minutes per person
When we have 7 people, then the number minutes they take will be 7*4/7=4 minutes per wall
4 min per wall for 4 walls then total time will be 4*4=16 minutes