Let x = pounds after the first pound = additional pounds
Set your equations to be equal.
5 + 0.3x = 7 + 0.2x
5-7 = 0.2x-0.3x
-2 = -0.1x
Solve for x.
Plug back in to get price.
$5 + $0.3/lb * 20lb = $5 + $6 = $11
Remember, x = additional pounds = number of pounds AFTER the first pound (meaning total pounds shipped = x+1 = 21lbs)
So, when you ship 21 pounds, you will pay $11 at either place.