Answer: 23 kg
Explanation:
Let's begin by using algebra to solve the problem.
Let x be the number of kilograms of salmon the fish-shop owner bought at the market.
We know that the total cost of the salmon was $400, so we can write:
400 = x * c
where c is the cost per kilogram of salmon.
We also know that all except for 2 kg of the fish were sold at a price per kg that was $10 more than what the owner paid at the market. So the price per kilogram of salmon at the fish shop was c + 10.
The total revenue from the sale of the fish was $540, so we can write:
540 = (x - 2) * (c + 10)
Now we can use these two equations to solve for x.
First, we can use the first equation to solve for c:
c = 400 / x
Then we can substitute this expression for c into the second equation:
540 = (x - 2) * (400/x + 10)
Simplifying this equation:
540 = 4000/x + 10x - 20 - 80/x
Multiplying both sides by x:
540x = 4000 + 10x^2 - 20x - 80
10x^2 - 20x - 4600 = 0
Dividing both sides by 10:
x^2 - 2x - 460 = 0
We can solve for x using the quadratic formula:
x = [2 ± sqrt(4 + 4*460)] / 2
x = [2 ± 44] / 2
Discarding the negative solution, we get:
x = (2 + 44) / 2
x = 23
Therefore, the fish-shop owner bought 23 kilograms of salmon at the market.