This year's "Yingdong Cup" calligraphy and painting competition in the city, our county sent three kinds of works to the competition, including 20 Chinese paintings, and the selected watercolor paintings were Chinese paintings (3/4) and calligraphy works (5/7) , How many calligraphy works were selected for the competition?
今年市"英东杯"书画竞赛，我县选送了三种作品参赛，其中有20幅国画，选送的水彩画是国画幅数的(3/4)，同时又是书法作品的(5/7)，选送了多少幅书法作品参赛？
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">(3/4) / (5/7)</gadget>
<output>21/20 = around 1.05</output>

<gadget id="calculator">20 * (21/20)</gadget>
<output>21</output>

<result>21</result>