Mr. Wang bought some basketballs for the school. He bought 15 basketballs for the first time, and 29 of the same basketballs for the second time. The difference between the two payments was 641.2 yuan. How much did Mr. Wang pay for the first time?
王老师为学校购买一些篮球，第一次买回15个，第二次买回同样的篮球29个，两次付的钱数相差641.2元，王老师第一次付了多少元？
<gadget id="calculator">29 - 15</gadget>
<output>14</output>

<gadget id="calculator">641.2 / 14</gadget>
<output>45.8</output>

<gadget id="calculator">45.8 * 15</gadget>
<output>687</output>

<result>687</result>