Xinhua Sporting Goods Factory is going to produce 1945 boxes of table tennis balls. Some of them have already been produced, and the rest will produce 85 boxes per day. It will take 13 days. How many boxes have they produced?
新华体育用品厂，准备生产1945盒乒乓球，已经生产了一些，剩下的每天生产85盒，还需13天，他们已经生产了多少盒？
<gadget id="calculator">85 * 13</gadget>
<output>1_105</output>

<gadget id="calculator">1_945 - 1_105</gadget>
<output>840</output>

<result>840</result>