How does Scheme handle elements in a list?

In the scheme, when a list is defined as ((item number a unit price a purchase number a) (item number b unit price b purchase number b) ...),
How should I describe the function to find the sum of the unit price of this number of purchases?

I know how to pass the list to the arguments of the function, but I don't know how to write how to take out the list and calculate the sum.