Let us say that,
A = perimeter of football field of the 1st school
B = perimeter of football field of the 2nd school
C = perimeter of football field of the 3rd school
So that:
A = 320 m
B = 360 m
C = 400 m
So the first thing we have to do to find for the minimum laps to make is to divide every perimeter by the smallest one, therefore divide all by 320 m, hence:
A’ = 1
B’ = 1.125
C’ = 1.25
Now multiply everything by a number such that all laps will be a whole number, in this case multiply everything by 8:
A’ = 8
B’ = 9
C’ = 10
Hence the laps to run are:
8, 9 and 10