Finding population one and four years ago with 2.7% annual growth

[Peter G.]

The population of a country is 15.2 million and is growing at a steady rate of 2.7% annually.
a) What was the population one year ago
b) What was the population four years ago

So I did this:

U_n+1 = U_n-1 * ((n-1)*0.973)

Doing so, I got for the first answer 14,789,600 but the book got 14,793,187

What did I do wrong?

 

[Filip Larsen]

Your equation is not correct. The basic relation is U_n+1 = U_n (1+2.7%), so for a) you can solve this to get U_n as a function of U_n+1.