Hey there! Hello!
The way I usually do these problems (there are most likely others) is to find the price per unit (your CDs in this example), then multiply that price by the amount that I need to figure out.
In this case, 2 CDs costs $15.00. To find the price of 1 CD, we'd need to divide the price by 2 to get $7.50 for each CD. Now that we have the price per unit, we can multiply by the amount needed, 15 CDs, to get the total. 7.50 times 15 is 112.5, meaning that 15 CDs will cost $112.50 total.
Hope this helped you out! Feel free to ask me any additional questions if you have any. :-)
