I'm getting ready for my next exam Thursday and I've got a hold of the regular solution problems but I can't figure this one out.

A tank contains 600 gallons of a 15% HCL solution. How many gallons must be drained and then replaced with pure HCL in order to obtain a solution that is 27% HCL.

I've tried this several different ways but I can't come up with 84.7059 drained and replaced with pure HCL as an answer.