The ratio of the number of people who subscribed to "Science Public" in Class 61 and Class 62 was 3:4. The two classes subscribed a total of 49 copies. How many copies did Class 61 subscribe to?
六一班和六二班订《科学大众》的人数比是3：4，两个班共订了49份，六一班订了多少份．
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">49 * (3/7)</gadget>
<output>21</output>

<result>21</result>