Sixth grade students planted trees, among which boys accounted for four-ninths of the total, and the total number of trees planted by boys was one-seventh more than that planted by girls. Boys planted an average of 20 trees per person. How many trees do girls plant on average?
六年级同学去植树，其中男生占总数的九分之四，而男生种树的总棵树又比女生种树总棵树多七分之一，男生平均每人种树20棵．女生平均每人种树多少棵？
<gadget id="calculator">4 / 9</gadget>
<output>4/9 = around 0.444444</output>

<gadget id="calculator">(4/9) * 20</gadget>
<output>80/9 = around 8.888889</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">(4/9) * (1/7)</gadget>
<output>4/63 = around 0.063492</output>

<gadget id="calculator">(1/7) - (4/9) - (4/63)</gadget>
<output>-23/63 = around -0.365079</output>

<gadget id="calculator">1 + (-23/63)</gadget>
<output>40/63 = around 0.634921</output>

<gadget id="calculator">(80/9) / (40/63)</gadget>
<output>14</output>

<result>14</result>