Annuity question?

Carl will start higher education in 5 years. He has just informed his parents that he wants to go to a University that will cost $18,000 a year for 4 years. Anticipating his ambitions, his parents started investing $3,000 per year 5 years ago and will continue doing so for the next 5 years.
How much more will his parents have to invest each year for the next five years to have the right amount of money needed for his education? Using 10% as the appropriate interest rate throughout the question.

Well, at that rate in 10 years, which is the amount of time they have, their investment will be at about $48,000, so they will be about $24,000 short. So my calculation would be that they need to invest approximately another $1500 a year.