a) 1/4 is also 5/20 mah.. Now we can compare them fairly.
3km => 5/20 
5km => 9/20 
8km => 6/20 

Since they have the same denominator, we can use the numerator for ratio.
3km : 5km : 8km => 5 : 9 : 6 ###

b)
total distance by the 3km participant = 5u X 3 =15u
total distance by the 5km participant = 9u X 5 =45u
total distance by the 8km participant = 6u X 8 =48u
Total distance covered = 48u + 45u + 15u = 108u

108u X 4 = 8208
432u = 8208
u = 19

Total number of participants = 20u = 20 X 19 = 380 ###