The total capacity of the D disk of Mr. Li’s home computer is 28GB, and the ratio of the used space to the available space is 1:3. How many GB is the available space of the D disk?
李老师家电脑的D盘总容量是28GB，已用空间和可用空间的比是1：3，D盘可用空间的容量是多少GB？
<gadget id="calculator">1 + 3</gadget>
<output>4</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

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

<result>21</result>