Teacher Li's son weighed 3.2 kg at birth, and 28.4 kg after one year. How many kg does he grow on average every month?
李老师的儿子出生时体重是3.2千克，一年后体重是28.4千克，平均每月增长多少千克？
<gadget id="calculator">28.4 - 3.2</gadget>
<output>25.2</output>

<gadget id="calculator">25.2 / 12</gadget>
<output>2.1</output>

<result>2.1</result>