So far, Mary has paid back $15 + $23 = $38 of the credit. So she still needs to pay $100 - $38 = $62.
This justifies what answer for what question? Q & A:
Mary does her grocery shopping on Saturday. She does her shopping only at a specific store where she is allowed a credit of $100, which must be paid in full before her next shopping trip. That week she spent the full credit limit and paid $15 of it on Tuesday and $23 of it on Thursday. How much credit will Mary need to pay before her next shopping trip?
62