Three pounds of squid can be purchased at the market for $18. Determine the equation and represent the function that defines the cost of squid based on weight.

From the information given,

3 pounds = $18

Therefore, 1 pound = 18/3 = $6 (this can be treated as a constant of proportionality) such that,

Cost, C = 6*Weight, W (pounds)

Equation;

C = 6W, where C = Cost in $, and W = Weight in pounds.