It would be 20. All you need to do is multiply by 2.
It would be zero because any expression with a zero in it is usually 0!
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.