There is a cuboid water tank with a water depth of 30 cm. After a cube with a side length of 1 decimeter is placed, the water level rises by 0.5 cm. After a watermelon is placed, the water level rises by 3.5 cm. What is the volume of the watermelon? ?
有一个长方体的水箱，水深30厘米，放入一个棱长1分米的正方体石块后，水面升高了0.5厘米，放入一个西瓜后，水面升高了3.5厘米，这个西瓜的体积是多少？
<gadget id="calculator">3.5 / 0.5</gadget>
<output>7</output>

<gadget id="calculator">1 * 1 * 1 * 7</gadget>
<output>7</output>

<result>7</result>