Answer: We can use the concept of rates to solve this problem.
A rate is a ratio of the amount of work done to the amount of time it takes to do the work. We can find the rate of work for the 4 painters by dividing the number of rooms they painted by the number of hours it took them:
Rate = Rooms painted / Hours
Rate = 12 rooms / 8 hours = 1.5 rooms per hour
Now we can use this rate to find out how long it will take 7 painters to paint 21 rooms. To do this, we'll use the formula:
Time = Rooms / Rate
Time = 21 rooms / 1.5 rooms per hour = 14 hours
So it would take 7 painters working at the same efficiency as the 4 painters 14 hours to paint 21 rooms.
Explanation: