Adding Two Multiples of Ten Such As 30 Plus 40

This idea walks around your house all day. Counting stacked crates of ten works exactly this way. Three stacks plus four stacks is seven stacks. This is worth learning because small facts you already know handle much larger numbers. 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. Adding tens is just like adding ones, only bigger. Three tens plus four tens is seven tens, which is 70. If you know three plus four is seven, you already know 30 plus 40.

Say it in tens as you work: three tens, four tens, seven tens. Build it with rods and count the rods, not the cubes. There are no loose ones to worry about here, which makes these problems quick.

Try saying it in tens out loud as you work. Three tens plus four tens is seven tens. Hearing the word tens keeps you from accidentally writing seven instead of seventy, which is the usual slip with these problems. Try a few on your own, and be patient with yourself.