Increasing Contribution. Usually, contributions are a percentage of your salary, which increases over time. Consider a $250 per month contribution, increased by 5% per year for 40 years at the same 8% rate, compounded continuously. In this case, how much money will be in your IRA at retirement? Note: R(t) will no longer be constant in this case it will depend on t. Your first step here is to figure out what R(t) is. Remember, to increase a number by 5%, you can multiply it by 1.05.