Answer:
A convenient formula to use is
S = ((1 + i)^n - 1) / i where S is the value of 1$ deposited for n periods at an interest rate of i
in this case n = 12 * 28 = 336 periods of deposit at an interest rate of
.0021 / 12 = .00175 = i
S = (1.00175^336 - 1) / .00175 = 456.8338 the value of 1$ after 336 periods
350 * 456.8338 = 159891.81 the value of 350 deposited monthly
Note that 350 * 336 would be 117,600
One must be careful to distinguish the above formula from
(1 - (1 + i)^-n) / i which gives the value of 1$ when the borrower is "paying" an interest rate of i - this would be the case for a mortgage - or what is the value of 1$ paid for n periods when paying an interest rate of i
