.
Sorry,
in your post, this sentence
"The contribution to profit and fixed cost from the sale of each book is $6.25."
does not make sense.
Double check with your source.
When you will find an error, fix it.
After that, you may re-post your request to the forum.
If you do, then PLEASE do not post it to me personally.
Have a nice day / evening / night / morning / afternoon . . .