distance = rate×time

Let w be the water current rate

(0.5 + w)t = 15

(0.5 -w)t = 5

-------------------- Add the two equations to get the time, t.

t = 20 seconds

0.5(20) + 20w = 15

10 + 20w = 15

20w = 5

w = 0.25 meters per second

The water current rate is 0.25 meters per second and it takes 20 seconds for the goldfish to swim 5 meters upstream against the current.