Why are time fences so important?

You wish to retire in 13 years, at which time you want to have accumulated enough money to receive an annual annuity of $23,000 for 18 years after retirement. during the period before retirement you can earn 9 percent annually, while after retirement you can earn 11 percent on your money. what annual contributions to the retirement fund will allow you to receive the $23,000 annuity? use appendix c and appendix d for an approximate answer, but calculate your final answer using the formula and financial calculator methods.