Find the savings plan balance after 15 months with an APR of 11% and monthly payments of $250.

Interest rate multiplier each month is 1 + 0.11/12 = 12.11/12 := m

Balance, B = $250(m^15 + m^14 + ... m^2 + m^1).

mB = $250(m^16 + m^15 + ... m^3 + m^2).

mB - B = $250(m^16 - m).

B = $250(m^16 - m) / (m-1)

B ≈ $4,037.12