In a school there are 697 girls and the rest are boys. If there are 228 more girls than boys how many boys are there in that school?
There are $g$ girls in that school.

There are $b$ boys in that school.

There are $s$ students in that school.

We know that $s=g+b$.

Now we have two equations and two unknowns. We can solve this system.

$s=g+b$

$s=100$

$100=g+b$

$100-g=b$

$b=100-g$

We substitute the value of $b$ into the first equation.

$s=g+b$

$s=g+100-g$

$s=100$

We have only one equation with one unknown. We can solve it.

$s=100$

$100=100$

$g=0$

$b=100$

There are 100 boys in that school.