As adults we forget that just quickly finding the answer for 10 more or 10 less than a specific number can be a feat of mental gymnastics for children. For your first or second grade class this is a great way to get their mental juices going.
Write a number at the board and let them give you the number that is 10 more. Then have them provide the number that is 10 less than the original number. After they’ve practiced this a number of times, it’s time to bring the numbers up to three digits and for each number have them come up with 10 more, 10 less, 100 more, 100 less of the number you begin with.
Also, giving them the number by speaking it will be much more difficult for them than seeing the written number. If you say “Three hundred fifty-seven,” they need to translate it to 357 in numbers before they can begin their process of mental math. Even more mental agility will be required if you say the number and instead of having them speak their answers, have them write their answers on a worksheet. They should find their answers using mental math– no fingers or calculators!
If you supply a worksheet, have students do this once a week as a warm-up and keep making the time shorter and shorter. As long as you keep it for fun and not for testing, they’ll enjoy being timed and doing it as fast as possible!