There are 50 red balls, 20 white balls and 20 black balls in the box, only touch one ball at a time, and then put it back, touch 10 times, what is the probability of touching white balls and black balls? Please explain why.
盒子里有50个红球、20个白球和20个黑球，每次只摸一个球，然后放回，摸10次，摸到白球和黑球的可能性如何？请说明理由．
<gadget id="calculator">50 + 20 + 20</gadget>
<output>90</output>

<gadget id="calculator">20 / 90</gadget>
<output>2/9 = around 0.222222</output>

<result>2/9 = around 0.222222</result>