This can be solved using a system of equations.

Subtract y from both sides.

Substitute:

Subtract 80 from both sides:

Divide both sides by 5:

Substitute.
#3 is correct at 120.4
#4 is 15/8...because 3/4+3/4=6/4 (for 20 doors) plus 3/8 (which is half of 3/4 for 5 more doors) equals 15/8 gallons or 1.875 gallons
#5 is $812.50 because if 50 shares cost $125, 325 shares (125×6.5) equals 812.50
There 60 pints in 15 gallon