It takes 30 minutes for Xiao Ming to go to school by car and walk home after school. If it takes 10 minutes to go back and forth by car. How many minutes does it take to walk back and forth.
小明上学时坐车，放学回家步行，一共要用30分钟．如果往返都坐车要10分钟．往返都步行要多少分钟．
<result>50</result>