Fraction Formulas

Ever wonder which fraction problems need inverted and multiplied, or which ones call for finding the least common denominator before you can do anything else? Keeping track of the procedures used for adding, subtracting, multiplying and dividing these fractions can be tricky for grade schoolers (and their parents!). Here’s some help!

But first a plug! We love our Professor B Math! It’s a contextual way to learn all aspects of math from a practical viewpoint in the least amount of time possible, and at your own pace. In spite of being a straight A student, I never learned math this way in grade school…and consequently, I feel like it’s finally making sense to me these past few years of teaching Professor B’s methods to my children.

So my daughter is learning ALLLL about fractions and we got the coolest fraction “tips” in today’s lesson.

Here you go for the next time your child is stumped as to which procedure to use for which fractional operation…

Know the code:

f=fraction, MN=mixed number, LCD=least common denominator

When the first formula says “f+f = LCD then add” it’s a short hand way of saying: “fraction plus fraction equals finding the least common denominator, then adding the fractions”

Okay…for the formulas:

  • f+f=LCD then add
  • f-f=LCD then subtract
  • fxf=reduce (if possible), then multiply across
  • f (divided by) f=invert and multiply
  • MN+MN=LCD then add
  • MN-MN=LCD then subtract
  • MNxMN=set up fraction, then multiply fraction
  • MN (divided by) MN=set up fraction then divide by fraction

If you drill your children on these formulas, and keep a “cheat sheet” taped to the inside of their mathbooks, eventually they won’t have to think very hard about which formula applies to their various fraction pursuits!

Gotta love it!

Professor B says whatever you do, if you learn your fractions backwards and forwards algebra will be a lot less intimidating!

Hope this helps some harried mom out there with her child’s homework!