The canteen sold a total of 328 math books in four days, and 55 books on the fourth day. How many books were sold on average in the first three days?
小卖部四天共卖出数学本328本，第四天卖出55本，前三天平均卖出多少本？
<gadget id="calculator">328 - 55</gadget>
<output>273</output>

<gadget id="calculator">273 / 3</gadget>
<output>91</output>

<result>91</result>