Answer:
56 gallons
Step-by-step explanation:
When her gas tank is full, she has x gallons in the tank.
when she went to the gas station, she had (5/8) of x gallons of gas in her tank. she added 21 gallons into her tank, then she had x gallons in her tank.
(5/8)x + 21 = x
subtract x and 21 from both sides
(5/8)x - x = -21
factor out an x from the left side
x(5/8 - 1) = -21
simplify the parenthesis
x(-3/8) = -21
multiply both sides by -8/3
x= 56 gallons
