4: The former term of 9 is multiplied by 9, to keep the ratio unchanged, how much should be added to the latter term
4：9的前项乘9，要使比值不变，后项应加上多少
<gadget id="calculator">9 * 9</gadget>
<output>81</output>

<gadget id="calculator">81 - 9</gadget>
<output>72</output>

<result>72</result>