How to find the original population if it increases with a constant rate?

Question

The population increases by 5% every year. What was the population in 1982, if in 1985 it was 1,85220?

My working:

     population in 1985 = 1,85,220
     rate=5%
     time = 3yrs

  ( A=P(1-R/100)^n )
     therefore, population in 1982 = 185220(1 - 5/100)^3
                          = 158802.9975

obviously that's wrong, but where's the problem?

Answer 1

You are making a basic error: If you increase a quantity $P_1$ by $y\%$ of itself to obtain the new value $P_2$, then decreasing $P_2$ by $y\%$ of itself does not get you back to $P_1$ (because you also decrease by $y\%$ of the increase previously gained). For example, increasing $100$ by $10\%$ of itself gives you $110$, but decreasing $110$ by $10\%$ of itself gives you $99$.

You need to start with the 1982 value, call this $x$, and then find an equation in $x$ and solve. William's answer explains how to do this.

(Incidentally, why do you say your answer is "obviously wrong"?)

Answer 2

To increase by $5$ percents is to multiply by $1.05$. This occurred for 3 years. So you obtain the equation:

$(1.05)^{3}x = 185220$

Solve for $x$ and this is the population in 1982.