Greg has lent his sister Tessa money on several occasions. Greg writes down all of his sister's debts in a notebook, the first time he lent her $40. A week later, Tessa paid him back half of her debt. A few days later she asked him for $10 more. How much does Tessa still owe Greg?
After Greg lent her $40, Tessa paid him back 40 / 2 =$<<40/2=20>>20.
So Tessa's debt was 40 - 20 = $<<40-20=20>>20.
Finally, she asked for $10 more, leaving the debt at 20 + 10 = $<<20+10=30>>30.
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