The ratio of the number of boxes of pears to apples shipped by the fruit shop is 7:4. The number of boxes of pears shipped is 21 more than that of apples. How many boxes of apples are shipped?
水果店运来梨和苹果的箱数比是7：4，运来的梨比苹果多21箱，运来苹果多少箱？
<gadget id="calculator">7 - 4</gadget>
<output>3</output>

<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<result>28</result>