If there are 623 students and the ratio of girls is 2 to 3 boys. how many girls are there?

The best way to solve this is to develop one or more equations and then solve for the unknown numbers.

I solved this problem twice using different sets of equations to make sure I was right, and here is what I found:

1) I used the equations x + y = 623, and x = (2/3)y. In these equations, x is the number of girls and y is the number of boys.
Substituting the second equation into the first, I get (2/3)y + y = 623.
Adding the left side together I get : (5/3)y = 623.
Dividing both sides by (5/3) I get 373.8.
This means that x (the number of girls) is 249.2.

2) For the second attempt, I developed the equation: 2x+3x=623. Here 2x is the number of girls, and 3x is the number of boys.
Adding the left side I get: 5x = 623.
Dividing both sides by 5 I get: x = 124.6.
This means that the number of girls (2x) is 249.2. Just like in the first method.

But since you can't have a 1/5 of a girl, the answer must be 249 girls.

There are roughly 3,460,254,000 girls as of April 30.