The accumulated value A of a monthly savings plan over a 20-year period is a function of the monthly contribution R and the interest rate r%, compounded monthly, according to

(a) Find the accumulated value of a plan that contributes $100 per month with interest rate 6%.
(b) Interpret f (250, 7.8) ≈ 143,648.
(c) Interpret ∂A / ∂r (250, 7.8) ≈ 17,770.
(d) Find ∂A / ∂R (250, 7.8) and interpret the result.

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240 1200R (1+ 1200, A = f(R, r)