Each table has two boxes. The box on top is the numerator and the box at the bottom is the denominator. Therefore, each table represents a ratio

Enter a ratio with two values in either box. Then enter only one value in the other box

Hit the calculate button and the fourth value will be computed!

The ratio of boys to girls is 2/5. How many boys are there in this classroom if there are 20 girls? This means that if there are 2 boys in this class, we have 5 girls.

Notice that number of boys is on top and number of girls is at the bottom

Therefore, pick a table and put 2 in the box on top and put 5 in the box at the bottom.

Next, since 20 represents number of girls and this number was at the bottom in the ratio of boys to girls ( 2/5 ), it will go in the box at the bottom in the other table

Hit calculate and you will see that there are 8 boys for 20 girls.

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