The sum of 7.5 and 2.5 minus 3 times a number, the difference is 1, find this number?
7.5与2.5的和减去一个数的3倍，差是1，求这个数？
<gadget id="calculator">2.5 - 1</gadget>
<output>1.5</output>

<gadget id="calculator">7.5 + 1.5</gadget>
<output>9</output>

<gadget id="calculator">9 / 3</gadget>
<output>3</output>

<result>3</result>