There are 21 students who participated in the science and technology group, which is 25% less than those who participated in the painting group. How many people participated in the painting group? (Solve it with equations. Can you solve it with arithmetic?)
同学们参加科技小组的有21人，比参加绘画小组的少25%，参加绘画小组的有多少人？(用方程解答．你还会用算术方法解答吗？)
<gadget id="calculator">25 / 100</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">21 / (3/4)</gadget>
<output>28</output>

<result>28</result>