You can put this solution on YOUR website!
Let us call the cost of regular 'g' so the cost of premium is 'g+0.21'
Multiply the number of gallons by its cost per gallon for each type and add together which will equal $56.64 so:
6g + 9(g+0.21)=56.64
solve for g:
6g + 9g + 1.89 = 56.64
15g = 56.64-1.89 (when moving a number from one side to the other side of an equation change the sign)
15g = 54.75
divide both sides by 15 to find g:
15g/15 = 54.75/15
g = 3.65
So the cost of regular is $3.65 and therefore the cost of premium is g+0.21 = $3.86
6(3.65) + 9(3.86) = 56.64 !!!