Kindergarten question
How to teach subtraction in the kindergarten with the tens place values and in horizontal sequence, for eg, 20-5=? They know to count their fingers, from the smaller number to the bigger number, but when the fingers are not enough to count, then what?
I'm not sure kindergartners are necessary ready for this. I would ONLY do these types of problems with concrete help.
Get a 100-bead abacus. Instruct them to first "make" 20 or show 20 on the abacus, and then move 5 away. Then "see" how many are left.
This online abacus is also really good for illustrating such.
Once they've done 20 - 5 and many other problems, you can ask if they notice a similarity in these problems:
20 - 5
50 - 5
70 - 5
etc.
But if some don't, wait till 1st grade. Practicing problems that "cross the ten" without manipulatives, such as 23 - 5 or 71 - 9, can wait even till 2nd. I realize kids might be able to do them by counting down, but to learn effective strategies for solving these kind of problems, children need a good foundation of place value (tens and ones), and that doesn't come immediately.
I'd use addition problems first, and a little easier ones, such as
44 + 2
62 + 7
and so on. These can help cement the place value concept as well, if you approach them right, such as, "62 has 6 tens and 2 ones, and then you add 7 more ones. Can you do that on the abacus, or with your ten-bundles and sticks, etc.?
With subtraction, try problems that "stay" within the same ten, such as 65 − 3. The idea is to use concrete aids and help them see they don't need to "touch" the whole tens when subtracting, so it's really similar to doing 5 − 3.
But still, I feel even this is not essential for kindergarten but can wait till first grade.
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