Answer:
y = 0.20x + 12.5
Q)
Lets set the cost of the cost as the y-axis and the amount of fish as the x-axis. 12.50, a constant and the y-intercept, is the price when you don't have any fish.
0.2 is the slope and in this case, the amount of 1 fish.
0.2x + 12.5 is the total amount as every fish added adds 20 cents. 12.5 is the fishtank which you need to get before getting any fish.