A computer company buys a batch of computers, sells 25% of them in the first week, and sells 21 units in the second week. At this time, the ratio of the remaining computers to the number of computers sold is 2:3. How many computers are in this batch? ?
电脑公司购进一批电脑，第一个星期卖出25%，第二个星期卖出21台，这时余下的和卖出台数的比是2：3，这批电脑原有多少台？？
<gadget id="calculator">2 + 3</gadget>
<output>5</output>

<gadget id="calculator">3 / 5</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">25 / 100</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(3/5) - (1/4)</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">21 / (7/20)</gadget>
<output>60</output>

<result>60</result>