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!