There is a number whose (2/5) minus 4.2 is equal to its (3/10), find this number. (solution by equation)
有一个数，它的(2/5)减去4.2与它的(3/10)相等，求这个数．(用方程解)
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">3 / 10</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(2/5) - (3/10)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">4.2 / (1/10)</gadget>
<output>42</output>

<result>42</result>