How I get a guaranteed 15% return paying down my wife's student debt.

My wife has 35k+ in student loans at a 5% interest rate. She has been paying them off, at just above the minimum, by herself. I have been hesitant to help her mostly because i can make a higher rate of return in my investments.

However, I recently discovered how to make a 15% return on my money by paying down her debt. It's simple -- I give her the money to pay down her debts that I would have otherwise spent on gifts (for Xmas, Valentine's Day, Birthday, etc.), and multiply that number by 10.

So, for instance, I would have normally spent $400 on gifts for her for X-mas, but am instead going to give her $4000 to pay down her debt. Thus, by giving $4000, I am "making" $400 (10%) on that investment. Combined with paying down debt with a 5% interest, I am actually making a 15% profit on my $4,000.

Anyone else do this?