The driver of the bus filled the gas tank with gas by putting in 21 gallon. If the gas gauge was at five-eighths before she filled the tank, how many gallons does tank hold?

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