A supermarket sells balloons. It sells 255 balloons in the morning and 240 balloons in the afternoon. There are 345 balloons left. How many balloons are there?
某超市卖气球，上午卖了255个，下午卖了240个，还剩345个，原来有多少个气球？
<gadget id="calculator">255 + 240 + 345</gadget>
<output>840</output>

<result>840</result>