Question Video: Finding the Missing Denominator That Makes a Whole One Mathematics • Second Year of Primary School

Video Transcript

What number is missing? Three-what equals one.

Now when we say three-what as we read that question out loud, it might sound a bit strange, but we can see that what we’ve got here is a fraction. There’s the numerator or the top number. And then we’ve got a line, but we can see that the missing number is the bottom number in this fraction. So we don’t know if we’re talking about halves or thirds or quarters. That’s why we have to read the question as three-what. This is the number that’s missing.

How do we know what this missing number’s going to be? Well, the first thing that we can say is the fraction that we’re given is equal to one, one whole. And there’s something we know about fractions that are worth the same as one whole. Do you remember what it is? Let’s think about halves for a moment. How many halves equal one whole? Two-halves. What about quarters? How many quarters equal one? Four-quarters are the same as one whole, aren’t they?

And these two examples should remind us something really important about fractions. If the numerator, which is the top number, and the denominator, or the bottom number, are the same, the fraction is worth one whole. And we could have looked at lots and lots of different types of fractions. But in the two examples we’ve just seen, two-halves and four-quarters, can you see that the numerator and the denominator are exactly the same? Both of these fractions are worth one whole.

Now that we’ve reminded ourselves about this, let’s come back to the question because we know we’re looking for another fraction that equals one whole. And the only thing we know about our fraction is the numerator. And that’s three. And as we’ve just said, if this fraction is going to be worth one, we need the numerator and the denominator to be the same. So if the top number’s three, the bottom number must also be three. Three-thirds equal one. And we know this is true, don’t we? We know that any fraction where the numerator and the denominator are the same is worth one whole. So if the numerator is three, we also know the denominator must be three too. The missing number in this fraction is three. Three-thirds equals one.