Math: solving percents with cross-multiplication

As a math tutor, you encounter this topic often – especially with students in vocational training.

Many ways exist to tackle percents.  However, the advantage of the cross multiplication method is its usefulness in virtually any situation involving them. If you missed my blog entry introducing the cross multiplication method, find it here.

Example 1: What is 15% of 390?

The key with percents is to realize that, for instance, 15% means 15 over 100. That is,


We can now incorporate our cross mutliplication scheme:

15/100 = x/390

Of course, x represents the number we need to know.

Now we do the actual cross-multiplication:


Dividing out 100 from both sides, we get

15*390/100 = x

We discover that 15% of 390 is 58.5.

Some other uses of cross multiplication with percents will be covered in future posts. For now, go back to the sun:)

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

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