Expand the former term of 3:7 to 21, to keep the ratio unchanged, how much should be added to the latter term.
把3：7的前项扩大到21，要使比值不变，后项应该加上多少．
<gadget id="calculator">21 / 3</gadget>
<output>7</output>

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

<gadget id="calculator">49 - 7</gadget>
<output>42</output>

<result>42</result>