Hit a balloon with a toy gun. For each shot, you can get 5 points for hitting one. If you miss, you will deduct 2 points. Liangliang fired 20 shots, and you get a total of 51 points. How many shots did he miss?
用玩具枪打气球，每发一枪，打中一个可得5分，如果未打中扣2分，亮亮发了20枪，共得51分，他有几枪未打中？
<gadget id="calculator">20 * 5</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>

<result>7</result>