Koizumi went to the stationery store to buy pencils and exercise books. He has enough money to buy 20 pencils and 20 exercise books; or buy 10 pencils and 50 exercise books. Now he has bought 13 pencils, how many exercise books can he buy at most.
小泉去文具店买铅笔和练习册．他身上的钱够买20支铅笔和20本练习册；或者买10支铅笔和50本练习册．现在他买了13支铅笔，最多还能买练习册多少本．
<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<gadget id="calculator">21 + 20</gadget>
<output>41</output>

<result>41</result>