# Video: Choosing the Suitable Unit for the Length of an Object

Estimate the answer in centimetres and complete the following statement: The length of your bed is ＿. [A] 7 cm [B] 5 cm [C] 200 cm [D] 20 cm [E] 30 cm

02:21

### Video Transcript

Estimate the answer in centimetres and complete the following statement: The length of your bed is what. Seven centimetres, five centimetres, 200 centimetres, 20 centimetres, or 30 centimetres.

This question is based on real life. And of course, we all have different length beds. But no matter what length our bed is, only one of the five answers really makes sense. Let’s use our knowledge of real-life measurements to help us here. We know that one centimetre is around about the width of a finger. Our beds fit our whole body on, not just a few fingers. So we can cross out the first two answers. Seven centimetres and five centimetres are far too short.

What about 30 centimetres? What can we think of that’s 30 centimetres that will help us visualise how long this actually is. Perhaps at school or at home you might have seen one of the short rulers that we might use to draw lines with. These are often 30 centimetres long. Are these the length of a bed? Of course not. And 20 centimetres is even shorter than this. So we can cross off 20 centimetres as well. There’s only one possible measurement that we’re left with.

This is why although we may have different length beds, we could narrow it down. There’s only one sensible answer. We know that 100 centimetres is the same as one metre. And we can visualise what one metre is like because we can think of a one metre ruler. If 100 centimetres is one metre, then we know 200 centimetres must be worth two metres. And if we imagine two metre rulers end-to-end, this seems like a sensible estimate for the length of a bed. We’ve used our knowledge of the measurements in the different answers to narrow it down. And we’ve ended with a sensible estimate that we can complete the statement with. The length of your bed is 200 centimetres.