Answer:
y = 250 + 810x
Explanation:
Let
y = total charges
x = cost per hour
Sally: building
y = 100 + 420x
Mia: painting
y = 150 + 390x
Find their total charges of building and painting
Add the total charges of both of them
y = 100 + 420x + 150 + 390x
y = 250 + 810x