Population decreasing rate is
18% monthly
Actual population = 500
Then
In 1 month decreases (500/100)• 18 = 90
Population = 500-90= 410
No find (410/100)•18 = 73.8
410-73.8= 336.2
In 3 months
(336.2/100) •18 = 60.5
336.2 - 60.5 = 276 fishes
ANSWER IS 276 fishes remain