Answer
$=25$ years.
Population $=9,900,000$.
Work Step by Step
Let the number of years after $2000$ be $=t$.
The population of Greece was $=10,600,000$.
Population decrease per year $=28,000$ people.
Population after $t$ years
$=10,600,000-28,000(t)$
The population of Belgium was $=10,200,000$.
Population decrease per year $=12,000$ people.
Population after $t$ years
$=10,200,000-12,000(t)$
Equate both populations for the condition of same population.
$\Rightarrow 10,200,000-12,000(t)=10,600,000-28,000(t)$
Add $-10,200,000+28,000(t)$ both sides.
$\Rightarrow 10,200,000-12,000(t)-10,200,000+28,000(t)=10,600,000-28,000(t)-10,200,000+28,000(t)$
Simplify.
$\Rightarrow 16,000(t)=400,000$
Divide both sides by $16,000$.
$\Rightarrow \frac{16,000(t)}{16,000}=\frac{400,000}{16,000}$
Simplify.
$t=25$ years.
Substitute the value of $t$ into the population of Belgium.
$=10,200,000-12,000(25)$
Simplify.
$=10,200,000-300,000$
$=9,900,000$
