In a subtraction formula, the sum of the minuend, the subtrahend and the difference is 140, and the ratio of the difference to the subtrahend is 3:4. What is the subtrahend?
在一个减法算式中，被减数、减数与差的和是140，差与减数的比是3﹕4，减数是多少．
<gadget id="calculator">140 / 2</gadget>
<output>70</output>

<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">70 * (4/7)</gadget>
<output>40</output>

<result>40</result>