This free math video shows another way to do mental subtraction. Basically, in subtraction problems you can add or subtract the same amount to both numbers without changing the difference. Cuisenaire Rods prove this to be true in a visual and kinesthetic way. It is always best to show children (and adults) WHY a rule works instead of just telling them the rule.
This free math video was kind of an experiment to see if my 8-year-old could assimilate all the information she’d learned thus far about fractions into adding and subtracting fractions with unlike denominators. I DO NOT want to give her rules until she has internalized the concepts, and, actually, I would prefer that she come up with the rules on her own when at all possible. That is why you will not see me straight up telling her what steps to take. I ask her what she thinks she should do, I give her a hint or the first step, but I try hard not to jump ahead to a “formula” too quickly. Understanding math concepts means understanding for life. Memorizing algorithms means forgetting in 10 or 20 or 30 years and feeling dumb. I’d prefer my children to actually understand this stuff!
Adding and subtracting fractions with Cuisenaire Rods looks exactly the same as adding whole numbers with the rods. The difference is in what rod you are calling “one.” Knowing how to add, subtract, multiply and divide whole numbers with the Cuisenaire rods makes it easy for her to do the same with fractions.
Once you are familiar with the first few methods for doing mental math, you may find that you enjoy this one the best of all!
If you are teaching mental math to children, give them many problems of one type until they become proficient in that particular way of doing mental math. Then show them another way until they can do it in their heads – without manipulatives. Keep doing this until you have shown them all the methods or until they are quick enough with one method that they don’t need others. Let them choose which method they prefer. None is better than any other.
Most children can be taught mental math strategies once they have their facts to 10 memorized – somewhere between Kindergarten and 2nd grade. Mental math is an excellent way to build and ensure number sense!