The number of boys in a certain class accounts for 40% of the whole class, and later 10 girls are transferred away. At this time, the number of boys accounts for 50% of the whole class. How many boys are there in this class?
某班男生人数占全班的40%，后来又转走10名女生，这时男生人数占全班的50%．这个班有男生多少人？
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">50 * (2/5)</gadget>
<output>20</output>

<result>20</result>