### Video Transcript

Find the missing number. One plus what equals four.

We can see two missing spaces
here. To begin with, we’re given an
addition equation. And instead of the missing number
being at the end like it often is, we can see that one of the numbers we’re adding
is missing. One plus what equals four. And underneath this equation, we
can see the same addition shown using pictures. One ball plus how many balls equals
four balls. The number that’s missing in this
question is one of the parts. To help us understand this, we
could sketch a part–whole model. Now we know the whole amount
because the answer to the addition or the total is four. This whole amount is what happens
when we add two parts together.

What else do we know? Well, we also know that one of the
parts is worth one. So one is a part, another number is
a part, and the whole amount is four. How can we find the missing
part? Well, we could count on from the
part we already know. Let’s use cubes to help us do
this. This cube represents the part that
we already know, which is worth one. Now let’s count on until we get to
four cubes, two, three, four. We’ve counted on three more cubes
to make a chain of four cubes. So we can complete our part–whole
model using the number three. One and three go together to make
four. Four is equal to one and three. Our missing number is three.