Tina pet sits to earn extra money. she charges a flat service fee of $20, plus $20 per day. if one of her customers spent less than $80, which of the following inequalities could be used to solve for x, the number of days the customer paid for pet sitting?
$20x + $20 < $80
$20x < $80
$20x - $20 < $80
$20x < $100