If a number is subtracted from 30 and then multiplied by 0.4, the product is 100. What is this number?
一个数减去30后再乘0.4，积是100，这个数是多少？
<gadget id="calculator">100 / 0.4</gadget>
<output>250</output>

<gadget id="calculator">250 + 30</gadget>
<output>280</output>

<result>280</result>