Led "d" be the distance covered by Peter;  Then the distance covered by Ben is   (20 + (20-d)) = 40-d kilometers.


The sum of the distances they covered before they met each other was  d + (40-d) = 40 kilometers.


Since they spent equal time from the start to the meeting point, Ben covered 4 times the distance covered by Peter.


So we have


    d + 4d = 40 km;


Hence, the distance, covered by Peter is    of 40 kilometers, i.e. 8 kilometers.   ANSWER