How can bridging through 10 help me to add and subtract?

NARRATOR: Hello Snoot, hello Fin.

What a great sand castle!

SNOOT: (ALIEN LANGUAGE)

NARRATOR: You're going to decorate it as well. What with?

SNOOT: (ALIEN LANGUAGE)

NARRATOR: Lesser spotted splarkle shells.

Very decorative.

How many shells do you have?

You've got 5 splarkle shells Snoot, and Fin, you've got 7.

How many shells is that altogether?

SNOOT: Er… Oh…

NARRATOR: Don't worry Snoot.

A handy way of working out 5 add 7 is to make 10 first.

Take your 5 shells Snoot, and add 5 from Fin's 7, which equals 10.

That leaves 2 shells left over.

So just add 2 to 10 and what do you get?

12 shells all together.

FIN: (ALIEN LANGUAGE)

NARRATOR: Oh right, Fin.

You only need 6 shells to decorate your sand castle.

That means you can take away 6 from 12 and just leave the rest.

How many does that leave?

FIN: (ALIEN LANGUAGE)

NARRATOR: OK Fin, you want to take away 6 from 12 by counting back through 10 as well.

12 minus 2 equals 10, so to get 10, just take 2 of the 6 shells away.

That leaves you with 4 shells to take away.

And what's 10 take away 4?

That's right, 6!

BOTH: Aha!

NARRATOR: But how do you know which shells to use and which to leave?

(POP)

NARRATOR: Ah… Maybe you should just leave them all?