Finding Ten Less Than a Number Without Counting Backward One by One

This idea walks around your house all day. Taking a dime out of a coin jar changes the dimes and leaves the pennies alone. This is worth learning because understanding tens replaces slow counting with instant knowing. 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.

This idea is simple once you see it. Ten less works the same way in reverse. Ten less than 58 is 48. Ten less than 90 is 80. Take away one rod and the cubes stay untouched, so only the tens digit drops by one.

If you find yourself counting back ten single steps, stop and look at the tens digit instead. Doing it the fast way is not skipping work. It is understanding what tens are.

Watch out at the edges. Ten less than 8 is not a whole number you can count with objects yet, so stay at ten and above for now. Knowing where a rule stops working is part of understanding the rule. Try a few on your own, and be patient with yourself.