To determine which fish Sasha bought, we need to calculate how much Sasha paid per pound for the fish.
First, we can calculate the price per pound of the fish Sasha bought. Since Sasha bought 3/4 lb of fish for $5.40, we find the price per pound like this:
\[ \text{Price per pound} = \frac{\text{Total cost}}{\text{Weight}} = \frac{5.40}{\frac{3}{4}} = 5.40 \times \frac{4}{3} = 7.20 \]
Now that we know the price per pound is $7.20, we can match this with the prices of the fish:
- Salmon: $9.60 a pound
- Whitefish: $7.20 a pound
- Swordfish: $8.40 a pound
- Haddock: $6.90 a pound
The fish that costs $7.20 per pound is whitefish.
So, the answer is whitefish.