Lets turn this into an equation:
30 is the cost per room, where x is the number of rooms. 100 is the constant fee, it will not change. Y is the total cost. This gives us:
y = 30x + 100
6 rooms means that x = 6 and we need to find y.
y = 30(6) + 100
y = 180 + 100
y = 280
This means that it will cost $280 to paint 6 rooms.
I hope this helps! :)
