There are 9 red balls and 50 yellow balls. The number of red balls remains the same. If the number of yellow balls is 6 times that of red balls, how many yellow balls should be added?
红球有9个，黄球有50个，红球的个数不变，要使黄球的个数是红球的6倍，黄球要加多少个？
<gadget id="calculator">9 * 6</gadget>
<output>54</output>

<gadget id="calculator">54 - 50</gadget>
<output>4</output>

<result>4</result>