Finding percentages with a double number line

We're told that Omar's class has 28 students in it. 21 of them take the bus to school. What percentage of the students in Omar's class take the bus to school? Pause this video and see if you can figure that out.

All right, well, I'm going to try to visualize this with what we call a dub and a double number line. So let's say that is 0, and 28 is the total number of students in this classroom right over here. It's a double number line because I'm going to make another number line right below it, but I'm going to write these points in terms of percentages. So 0 students would be 0 of the students in Omar's class, and 28 students would be 100 of the students in Omar's class.

And so what we really need to do now is we know that 21 of them take the bus. 21 is going to be roughly around here, so that's 21. So we really just need to figure out: what percentage is this going to be? One way to think about it is: what fraction is 21 of 28? Well, if I write 21 over 28, we know that we can divide both the numerator and the denominator by 7. They're both divisible by 7. 21 divided by 7 is 3, and 28 divided by 7 is 4. So 21 is 3/4 of 28.

So let's divide this number line from 0 to 28 into fourths. So that would be halfway, and now we have it divided into fourths. We can see that 21 is one, two, three of these fourths. Well, if 21 is three-fourths of the way to 28, then whatever percentage this is here, that would be three-fourths of the way to a hundred.

So let's divide this into fourths as well. Now we know that one-fourth of a hundred is twenty-five percent, two-fourths of a hundred is fifty percent, and three-fourths of a hundred is seventy-five percent. So what percentage of the students in Omar's class take the bus to school? 75 percent.