Grandma Zhang bought 2 kilograms of lychees and 3.5 kilograms of watermelon and paid 40.5 yuan; Grandma Li also bought the same 2 kilograms of lychees and 4 kilograms of watermelon and paid 42 yuan. How much is watermelon per kilogram?
张奶奶买了2千克荔枝和3.5千克西瓜，付了40.5元；李奶奶也买了同样的荔枝2千克和西瓜4千克，付了42元，西瓜每千克多少元钱？
<gadget id="calculator">42 - 40.5</gadget>
<output>1.5</output>

<gadget id="calculator">4 - 3.5</gadget>
<output>0.5</output>

<gadget id="calculator">1.5 / 0.5</gadget>
<output>3</output>

<result>3</result>