Teacher Wang reads a book, if he reads 28 pages a day, he can finish it in 15 days. I read 21 pages on the first day. According to the progress of the first day, how many days can I finish it?
王老师看一本书，如果每天看28页，15天可以看完．第一天看了21页，按照第一天的进度，几天可以看完？
<gadget id="calculator">15 / 21</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">28 * (5/7)</gadget>
<output>20</output>

<result>20</result>