Answer
Water speed: 5.5
Current speed: 1.5
Work Step by Step
Let call "w" be speed of water and "c" be speed of the current
Given: w-c=4 (headwind)
w+c=7 (tailwind)
Elimination method: (w-c)-(w+c)=4-7 => (w-w)-2c=-3 => c=1.5
w=7=1.5=5.5