Grandpa Song went to the street to buy vegetables. With all the money he brought, he bought potatoes, which could buy 15 kilograms; all he bought tomatoes, which could buy 10 kilograms. He first bought 9 kilograms of potatoes, how many kilograms of tomatoes can he buy with the remaining money.
宋爷爷上街买菜，他带的钱都买土豆，能买15千克；都买西红柿，能买10千克．他先买了9千克土豆，剩下的钱还能买多少千克西红柿．
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">9 * (1/15)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 - (3/5)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(2/5) / (1/10)</gadget>
<output>4</output>

<result>4</result>