Subtracting Multiples of Ten from Multiples of Ten

Think about where you have seen this before. Removing three crates from a stack of seven leaves four crates. The unit is bigger but the counting is the same. This is worth learning because one small fact handles both the small and the large version.

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.

Here comes a new math idea. Subtracting tens works like subtracting ones. Seventy take away thirty is seven tens take away three tens, which is four tens, or 40. Say it in tens while you work.

Build it with rods: lay out seven rods, take three away, and count the four that remain. Because there are no loose ones in these problems, nothing needs to be unbundled, which keeps the work quick and clean.

Try building it and taking it apart. Lay out seven rods, remove three, and count what is left. Watching the rods leave makes seventy minus thirty feel like something that happened rather than something you memorized. Try it once with someone beside you, then try it alone.