1 and 2 are the factors its a compatable number
In order to solve a proportion you need to cross multiply, so you would do:
4x20 and 9xX and you would get...
9x=80
then divide by nine to get
x=8.8888888889
which is technically 8.9
The line will flip across the y-axis.
Quantity of paint left =
gallons
Solution:
Quantity of yellow paint =
gallons
Quantity of green paint =
gallons
Quantity of blue paint =
gallons
Quantity of paint used =
gallons of each paint
Quantity of paint left =
gallons of each paint
Quantity of paint left = 
Taking LCM for 5, 6, 8, we get LCM = 120



Hence, Mark will have
gallons of paint left after painting the mural.