It takes Mary $$30$$ minutes to walk uphill $$300 $$ m from her home to school, but it takes her only $$10$$ minutes to walk from school to home along the same route. What is her average speed, in m/min, for the round trip? (adapted from 2003 AMC 10 Problem, Question \#4) 
Overseas Competition->Knowledge Point->Distance Word Problems->Distance Word Problems on Straight Road
Mary walks a total of $$300+300=600\rm$$ meters in $$40$$ minutes.  Average speed: $600\div40=15$ m/min 