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 x boys and y girls in a school. 

x + y = total number of students in the school

y = total number of girls in the school

697 = number of girls in the school

x = number of boys in the school

So we have the following system of two equations with two unknowns:

x + y = total number of students in the school

y = 697

Solve the system:

x + 697 = total number of students in the school

x = total number of students in the school - 697

x = total number of students in the school - y

Now we have an equation with one unknown:

x = total number of students in the school - 697

We know that there are 228 more girls than boys. So:

228 = y - x

y = x + 228

Now we have two equations with one unknown each. We can solve them:

x + y = total number of students in the school

x + x + 228 = total number of students in the school

2x + 228 = total number of students in the school