What function describes how fast the national debt is

Question

Qusetion: U.S. debt for selected years from 1900 to 2009, the national debt d, in billions of dollars, can be modeled by

d = 1.67e^0.0825t

where t is the number of years past 1900 (Source: Bureau of Public Debt, U.S. Treasury).

(a) What function describes how fast the national debt is changing?

(b) Find the instantaneous rate of change of the national debt model d(t) in 1950 and 2015.