Li Ming plays ball with an air gun. If he hits a shot, he will get 5 points, and if he misses a shot, 2 points will be deducted. He hit 20 shots and scored a total of 51 points. How many shots did he hit?
李明用气枪打球，打中一枪可得5分，未打中要倒扣2分．他打了20枪，一共得了51分，他打中了多少枪．
<gadget id="calculator">5 * 20</gadget>
<output>100</output>

<gadget id="calculator">100 - 51</gadget>
<output>49</output>

<gadget id="calculator">5 + 2</gadget>
<output>7</output>

<gadget id="calculator">49 / 7</gadget>
<output>7</output>

<gadget id="calculator">20 - 7</gadget>
<output>13</output>

<result>13</result>