A school has 800 pupils. If 400 are boys, how many girls are there?
The total pupils will equal the number of boys plus the number of girls. We have 800 total pupils. 400 of them are boys. And we need to know how many of them are girls. 400 plus what equals 800?
Let’s ask the question like this: four plus what equals eight? We know that four plus four equals eight. And that helps us solve this problem, because we can say 400 plus 400 equals 800. There are 400 girls at the school.