Yixintang is engaged in festival promotion activities. The unit price of a box of calcium tablets is 168 yuan. Buy three boxes and get one free. Xiao Ming’s mother bought 4 boxes. How much is the average discount per box?
一心堂搞节庆促销活动，一盒钙片单价168元，买三盒送一盒，小明的妈妈买了4盒，平均每盒优惠多少元？
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">168 * (3/4)</gadget>
<output>126</output>

<gadget id="calculator">168 - 126</gadget>
<output>42</output>

<result>42</result>