The formula for this calculation is Future Value = Payment • [((1 + I)n - 1)/I], where I = interest rate, and n = periods.
In this case, P = $800/month, n = 18 months, and I = 6% (0.06) per year.
Since the information involves months instead of years, it is necessary to divide the interest rate by 12 to obtain the monthly rate. So, 0.06 ÷ 12 = 0.005
We can then enter the information into our formula to obtain:
FV = $800 • [((1.005)18 - 1)/.005]
FV = $800 • [(1.0939 - 1)/.005]
FV =$800 • (0.0939/0.005)
FV = $800 • 18.785
FV ≅ $15,028