Problem 8.3. In the grove, there are 140 chameleons - blue and red. One day, several blue chameleons changed their color to red. As a result, the number of blue chameleons decreased by 5 times, and the number of red chameleons increased by 3 times. How many chameleons changed their color?
Answer: 80.

Solution. Let the number of blue chameleons become $x$. Then initially, there were $5 x$ blue chameleons. Accordingly, the number of red chameleons initially was $140-5 x$. Then the number of red chameleons became $3 \cdot(140-5 x)$. Since the total number of chameleons remained the same, we get the equation

$$
x+3 \cdot(140-5 x)=140
$$

Solving it, we find $x=20$. Then the number of chameleons that changed color is $5 x-x=4 x=4 \cdot 20=80$ chameleons.