75% of a number minus the product of 0.5 and 1.8, the difference is 2.1, find this number.
一个数的75%减去0.5与1.8的积，差是2.1，求这个数．
<gadget id="calculator">0.5 * 1.8</gadget>
<output>0.9</output>

<gadget id="calculator">0.9 + 2.1</gadget>
<output>3</output>

<gadget id="calculator">75 / 100</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">3 / (3/4)</gadget>
<output>4</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<result>4</result>