The good horse runs 120 kilometers every day, and the bad horse runs 75 kilometers every day. If the bad horse runs for 3 days first, how many days does it take for the good horse to catch up with the bad horse?
良种马每天跑120千米，劣种马每天跑75千米，若劣种马先跑3天，良种马需多少天追上劣种马．
<gadget id="calculator">75 * 3</gadget>
<output>225</output>

<gadget id="calculator">120 - 75</gadget>
<output>45</output>

<gadget id="calculator">225 / 45</gadget>
<output>5</output>

<result>5</result>