The fuel carried by a steamboat can be used for 6 hours at most. It travels 15 kilometers per hour downstream and 10 kilometers per hour upstream when it returns. How many kilometers does this steamboat need to go back?
一只汽船所带的燃料，最多用6小时，去时顺流每小时行15千米，回来是逆流每小时行10千米，这只汽船最多行出多少千米就需往回开？
<gadget id="calculator">15 * 2.5</gadget>
<output>37.5</output>

<result>37.5</result>