A fruit vendor once sold fruit worth 20 yuan, accidentally received a counterfeit 50 yuan bill, and found out 30 yuan in real money. How much did he lose this time?
一水果商贩一次卖了20元钱的水果，不小心收了一张面额为50元的假钞，又反找出30元真钱，这次共亏了多少元．
<gadget id="calculator">20 + 30</gadget>
<output>50</output>

<result>50</result>