There are 60 tons of stones, which have been transported twice by 6 vehicles with a load of 3.5 tons, and the remaining stones are transported at one time by a vehicle with a load of 4.5 tons. How many vehicles are needed?
现有60吨石子，已经用6辆载重为3.5吨的车运了两次，剩下的石子用载重4.5吨的车一次运完，还需要多少辆车．
<gadget id="calculator">3.5 * 6 * 2</gadget>
<output>42</output>

<gadget id="calculator">60 - 42</gadget>
<output>18</output>

<gadget id="calculator">18 / 4.5</gadget>
<output>4</output>

<result>4</result>