Three years ago, the population of Pirajussaraí was equal to the population that Tucupira has today. Since then, the population of Pirajussaraí has not changed, but the population of Tucupira has grown by $50 \%$. Today, the sum of the populations of the two cities is 9000 inhabitants. What was the sum of these two populations three years ago?
(a) 3600
(b) 4500
(c) 5000
(d) 7200
(e) 7500
The correct option is (e).

Let $p$ be the population of Tucupira three years ago. Since this population grew by $50 \%$, Tucupira currently has $p+50 \%$ of $p$ inhabitants, that is,

$$
p+\frac{50}{100} p=p+0.5 p=1.5 p \quad \text { inhabitants. }
$$

Since the population of Pirajussaraí did not grow in these 3 years and was equal to that of Tucupira 3 years ago, we can conclude that the current population of Pirajussaraí is $p$. Given that the sum of the populations of the two cities today is 9000, we get $p+1.5 p=9000$, hence $p=9000 / 2.5=3600$. Therefore, the sum of the two populations, three years ago, was $3600 \times 2=7200$ inhabitants.