A batch of watermelons has been brought in, and they are going to be sold in two categories. The big ones are 0.4 yuan per kilogram, and the small ones are 0.3 yuan per kilogram. The total value of this batch of watermelons is 290 yuan. If the price of watermelons per kilogram is reduced by 0.05 yuan, this batch of watermelons can only be sold 250 yuan, ask: How many kilograms of large watermelons are there?
运来一批西瓜，准备分两类卖，大的每千克0.4元，小的每千克0.3元，这样卖这批西瓜共值290元，如果每千克西瓜降价0.05元，这批西瓜只能卖250元，问：有多少千克大西瓜？
<gadget id="calculator">290 - 250</gadget>
<output>40</output>

<gadget id="calculator">40 / 0.05</gadget>
<output>800</output>

<gadget id="calculator">0.3 * 800</gadget>
<output>240</output>

<gadget id="calculator">290 - 240</gadget>
<output>50</output>

<gadget id="calculator">0.4 - 0.3</gadget>
<output>0.1</output>

<gadget id="calculator">50 / 0.1</gadget>
<output>500</output>

<result>500</result>