If you have both spreadsheet A and B on the same computer then you can use the VLOOKUP formula to figure it out.
Open both of the spreadsheets. On the spreadsheet with more items, insert a column next to the items and enter in the VLOOKUP formula. When you are done it would look something like what is below.
For a quick tutorial on VLOOKUP you can go to:
After you are done dragging this formula down to match all items, select the column, copy, paste special (selecting “values” as what you want to paste). With that same column selected, hit CTRL+H (or use the Home ribbon, Editing block, Find and Select, Replace…)
In the Find what box, enter “#N/A”
Leave the replace with box empty, and click Replace All.
Now sort by that column and you will find your mysterious 15.
Let us know how it goes. Good luck (This answer may sound long but it does not take a great amount of time)