Checking a Subtraction of Tens by Adding the Tens Back On

Look for this idea outside of your lessons. Bank tellers and shopkeepers count twice on purpose. The second count is not doubt, it is care. This is worth learning because checking is how careful people catch their own mistakes. Watch for it today, and when you spot it, tell someone what you noticed and how you knew. Ask yourself where else the same idea might be hiding, then go and look.

Let us think about this slowly. Check your tens subtraction the same way you checked small ones. If 80 minus 30 gave you 50, add 50 and 30 back together.

You get 80, exactly where you started, so the answer holds. If your check does not land where you began, something slipped, and it is far better to find that yourself than to hand in work with a hidden mistake in it.

Here is a caution about checking. Your check must land exactly on your starting number, not near it. Close is not correct in arithmetic. If your check misses by ten, you probably lost or gained a bundle somewhere. Show your work to someone and explain it in your own words.