30% of the pupils in a class are boys.there are 21 girls.how many pupils are there in the class in all?

.7 x = 21

x = 21/.7 = 30

To find out the total number of pupils in the class, we need to use the information given in the problem.

First, we know that 30% of the pupils in the class are boys. This means that the remaining 70% are girls since the total percentage must add up to 100%.

Next, we are told that there are 21 girls in the class. We can use this information to find out the total number of pupils in the class.

Let's set up an equation to represent the percentage of girls in the class:

70% = 21

To solve for the total number of pupils in the class, we can set up a proportion:

70/100 = 21/x

To solve for x (the total number of pupils in the class), we can cross-multiply:

70x = 2100

Now, divide both sides of the equation by 70:

x = 2100/70

Simplifying the equation gives us:

x = 30

Therefore, there are 30 pupils in total in the class.