As far as i can understand the question, it is given that Karen proofreads 15 pages. She gets a rate of $40 for proofreading 2 pages. The money Karen makes for proofreading 15 pages has to be determined.
Number of pages Karen proofreads = 15
Then
For proofreading 2 pages Karen receives = 40 dollars
So
For proofreading 15 pages, Karen receives = (40/2) * 15 dollars
= 20 * 15 dollars
= 300 dollars
So Karen receives $300 for proofreading 15 pages. I hope this is what you were looking for and the procedure for solving the problem is also clear to you.