..Im working through a book "Maths for Science" and here is another example I dont understand:

For example, can be simplified by dividing the numerator

and denominator by 3 to get Mr F says: The red 3 should be a 5 (since 5 is the common factor of 5 and 15).

This is what I dont understand how can the numerator be divided by 3? 5 divided by 3 is 1.666666667 on my calculator!

Although I instantly know is also in my mind.

$ilver