An electricity company charges its customers a fixed service charge each month, in addition to a variable charge based on the number of kilowatt-hours (kWh) consumed. In June, a household's bill was $78 for consuming 300 kWh. In July, their bill was $113 for consuming 500 kWh. What is the fixed monthly service charge?
$22.50
$24.00
$25.50
$27.00
$28.50
