Mrs. Stancil is planning to use wrapping paper to cover a gift box that is the
shape of a rectangular prism with dimensions of
3ft by 4ft by 5ft. If the wrapping paper costs
S0.05 per square foot, how much will it cost?

Work Shown:

x = amount of the 30% solution (in gallons)

y = amount of the 80% solution (in gallons)

The two amounts must add to 100 as this is the total we want, so x+y = 100 which becomes y = 100-x after subtracting x from both sides

The expression 0.30*x represents the amount of pure alcohol from the first batch, while 0.80*y represents the amount of pure alcohol from the second batch. In total, we have 0.30*x+0.80*y gallons of pure alcohol. We want 50 gallons of pure alcohol (50% of 100 gallons is 50 gallons), therefore we end up with this equation: 0.30*x+0.80*y = 50

Let's use substitution to isolate the variables.

0.30*x+0.80*y = 50

0.30*x+0.80*(100-x) = 50 ... replace y with 100-x

0.30*x+0.80*(100)+0.80*(-x) = 50 ... distribute

0.30*x+80-0.80x = 50

-0.50*x+80 = 50

-0.50*x+80-80 = 50-80 ... subtract 80 from both sides

-0.50*x = -30

x = -30/(-0.50) divide both sides by -0.50

x = 60

If x = 60, then y is...

y = 100-x

y = 100-60

y = 40