The monthly benefit B (in thousands of dollars) from a retirement account that is invested at 9% compounded monthly is a function of the account value V (also in thousands of dollars) and the number of years t that benefits are paid, and it can be approximated by

(a) Find the benefit if the account value is $1,000,000 and the monthly benefits last for 20 years.
(b) Find and interpret ∂B / ∂V (1000, 20).
(c) Find and interpret ∂B / ∂t (1000, 20).

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3V B = f(V, t) = 400 – 400e-0.0897t