A batch of watermelons is brought in, and they are going to be sold in two categories, the big ones are 2.6 yuan per kilogram, and the small ones are 1.8 yuan per kilogram. The total value of these watermelons sold in this way is 1260 yuan. If the price of watermelons per kilogram is reduced by 0.4 yuan, this batch of watermelons can only be sold for 1020 yuan. 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2.6元，小得每千克1.8元，这样卖这批西瓜共值1260元．如果每千克西瓜降价0.4元，这批西瓜只能卖1020元，问有多少千克大西瓜？
<gadget id="calculator">1_260 - 1_020</gadget>
<output>240</output>

<gadget id="calculator">240 / 0.4</gadget>
<output>600</output>

<gadget id="calculator">600 * 1.8</gadget>
<output>1_080</output>

<gadget id="calculator">1_260 - 1_080</gadget>
<output>180</output>

<gadget id="calculator">2.6 - 1.8</gadget>
<output>0.8</output>

<gadget id="calculator">180 / 0.8</gadget>
<output>225</output>

<result>225</result>