There are 21 large and small balls in the kindergarten, among which the number of small balls accounts for (5/7) of the total number of balls. After buying another batch of small balls, the number of small balls now accounts for (5/6) of the total number of balls. How many small balls did the kindergarten buy?
幼儿园有大、小皮球共21个，其中小皮球的数量占皮球总数的(5/7)．又买来一批小皮球后，这时的小皮球数量占皮球总数的(5/6)．幼儿园又买来了多少个小皮球？
<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">21 * (5/7)</gadget>
<output>15</output>

<gadget id="calculator">21 - 15</gadget>
<output>6</output>

<gadget id="calculator">5 / 6</gadget>
<output>5/6 = around 0.833333</output>

<gadget id="calculator">1 - (5/6)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">6 / (1/6)</gadget>
<output>36</output>

<gadget id="calculator">36 - 21</gadget>
<output>15</output>

<result>15</result>