It is very important to read the question carefully. There is chance of being mistaken with the way it has been represented. Also most of the information's required for getting to the solution is already given in the question. Based on those information's the answer to the question can be easily deduced.
Cost of each pound = $3.50
Then
The cost of 8 pounds = (8 * 3.50) dollars
= 28 dollars
So the cost of 8 pounds comes out to be $28. I hope there is nothing much complicated about the process and it is understandable.