Question 141110
let s=salmon and c=current


upstream __ s-c=100m/20 min __ s-c=5m/min


downstream __ s+c=100m/8min __ s+c=12.5m/min


adding the two equations __ s-c+(s+c)=5+12.5 __ 2s=17.5 __ s=8.75m/min


substituting __ 8.75+c=12.5 __ c=3.75m/min