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.
So, our friendly number will be 300: it's only two numbers away from 298!
We multiply 300 by 6: it's 1800. (3*6=18, and the 0s we can just add at the end)
now we only need to subtract those two numbers (mupliplied by 6): so we subtract 12.
the result will be 1800-12=1788.
Any number is divisible by 4, if the last 2 digits are divisible by 4,
Any number is divisible by 8, if the last 3 digits are divisible by 8, unless the last 2 digits are a multiple of 8