Assume that the price of 1 kg fish is x and 1 kg of prawns is y
∵ She has 70 dollars
∵ The price of 4 kg fish and 3 kg prawns will be the total money with her
∴ 4x + 3y = 70 ====== (1)
∵ She bought 3 kg fish and 4 kg prawns
∵ She has 3.50 dollars left
→ That means the price of them = 70 - 3.50 = 66.5 dollars
∴ 3x + 4y = 66.5 ===== (2)
Now we have a system of the equations to solve it and find the value of y
Multiply the first equation by -3 and the 2nd equation by 4
∵ -3(4x) + -3(3y) = -3(70)
∴ -12x - 9y = -210 ===== (3)
∵ 4(3x) + 4(4y) = 4(66.5)
∴ 12x + 16y = 266 ===== (4)
Add equations (3) and (4)
∴ 7y = 56
Divide both sides by 7 to find y
∴ y = 8
∴ The price of 1 kg of prawn is $8