The value of I is 4800.
I = (8000)(0.6)(1) = 4800.
Investment problems usually involve simple annual interest (as opposed to compounded interest), using the interest formula I = Prt, where I stands for the interest on the original investment, P stands for the amount of the original investment (called the "principal"), r is the interest rate (expressed in decimal form), and t is the time.
For annual interest, the time t must be in years. If they give you a time of, say, nine months, you must first convert this to 9/12 = 3/4 = 0.75 years. Otherwise, you'll get the wrong answer.
Investment word problems are not generally terribly realistic; in "real life", interest is pretty much always compounded somehow, and investments are not generally all for whole numbers of years.
To learn more, see PurpleMath.com's "Investment" Word Problems.