There are 42 students in the first class of the second grade, including 33 young pioneers. There are 20 boys in this class, and 4 of the girls are not Young Pioneers. How many of the boys are Young Pioneers?
二年级一班共42名同学，其中少先队员33人．这个班男生20人，女生中有4个不是少先队员，男生中有多少人是少先队员．
<gadget id="calculator">42 - 33</gadget>
<output>9</output>

<gadget id="calculator">9 - 4</gadget>
<output>5</output>

<gadget id="calculator">20 - 5</gadget>
<output>15</output>

<result>15</result>