7: Add 45 to the latter item of 15, how much should be added to the former item, so that the ratio remains unchanged.
7：15的后项加上45，前项应加上多少，比值才不变．
<gadget id="calculator">15 + 45</gadget>
<output>60</output>

<gadget id="calculator">60 / 15</gadget>
<output>4</output>

<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<gadget id="calculator">28 - 7</gadget>
<output>21</output>

<result>21</result>