###Human: 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?, ### Assistant: After Greg lent her $40, Tessa paid him back 40 / 2 =$20.
So Tessa's debt was 40 - 20 = $20.
Finally, she asked for $10 more, leaving the debt at 20 + 10 = $30. 
 So the answer is :  30