A school originally had 40 skipping ropes, of which the ratio of the number of short ropes to the number of long ropes was 5:3, and another batch of short ropes was bought, and the number of short ropes accounted for 75% of the total. How many short ropes to buy.
某学校原有跳绳40根，其中短绳根数与长绳根数的比是5：3，又买进一批短绳，这时短绳的根数占总数的75%．买进短绳多少根．
<gadget id="calculator">5 + 3</gadget>
<output>8</output>

<gadget id="calculator">3 / 8</gadget>
<output>3/8 = around 0.375</output>

<gadget id="calculator">40 * (3/8)</gadget>
<output>15</output>

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

<gadget id="calculator">1 - (3/4)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">15 / (1/4)</gadget>
<output>60</output>

<gadget id="calculator">60 - 40</gadget>
<output>20</output>

<result>20</result>