40% of a number minus 4.2 is equal to 30% of it. Find this number. (solution by equation)
一个数的40%减去4.2与它的30%相等，求这个数．(用方程解)
<gadget id="calculator">0.4 - 0.3</gadget>
<output>0.1</output>

<gadget id="calculator">4.2 / 0.1</gadget>
<output>42</output>

<result>42</result>