Xiao Ming brought 21 yuan to buy children's books, and Xiao Gang brought 15 yuan to buy the same book, each children's book is 4 yuan, how many books can they buy together?
小明带了21元去买儿童读物，小刚带了15元去买同样的书，每本儿童读物4元，他们合起来可以买多少本？
<gadget id="calculator">21 + 15</gadget>
<output>36</output>

<gadget id="calculator">36 / 4</gadget>
<output>9</output>

<result>9</result>