> This investment word problem is ridiculous?

This investment word problem is ridiculous?

Posted at: 2014-12-05 
How to do this?:

Suppose Mikey invests $15,000 at 6%. How much additional money should Mikey invest at 4% to produce a yield of 5.5% on the entire amount investested?

Please explain the steps to me, I always get this one wrong :(