The number of the school chorus is between 60 and 70, and the number of boys is equal to the number of girls (4/9). How many boys are there in the school chorus?
学校合唱队人数在60～70人之间，男生人数是女生人数的(4/9)．校合唱队男生有多少人．
<gadget id="calculator">9 + 4</gadget>
<output>13</output>

<gadget id="calculator">4 / 13</gadget>
<output>4/13 = around 0.307692</output>

<gadget id="calculator">65 * (4/13)</gadget>
<output>20</output>

<result>20</result>