Which of the following is the best estimate for the length of a pen? A) Twelve
kilometres, B) twelve centimetres, C) twelve millimetres, or D) twelve metres.
Now all of these measurements have been given in the metric system and the metric
system of length is based around the metre. And just to give you some idea, the average car is
about one point four, one point five metres tall. Now let’s think about all these different units which have been defined as being
multiples of a metre or fractions of a metre. A kilometre is a thousand metres. So twelve kilometres is twelve thousand metres. That’s thousands of times as tall as a car; that’s not a very good estimate for the
length of a pen.
Okay, how about twelve centimetres? Well, one centimetre is a hundredth of a metre. There’re a hundred centimetres in
a metre. So twelve centimetres is twelve hundredths of a metre, which is about a tenth of
a metre. And on our car that would be about the width of a headlight, which seems like a
reasonable estimate for the length of a pen.
Let’s think about twelve millimetres. One millimetre is a thousandth of a metre. And that means that one centimetre has ten millimetres in it. So twelve
millimetres is gonna be a tenth the size of twelve centimetres. And that’s probably about the width of a bulb from a headlight. Well that’s
pretty small for a pen. So that’s probably not the right answer.
And then lastly, we need to consider metres. Remember we said the height of an
average car is about one point five metres. So twelve metres is about the height of eight
average cars stacked on top of each other. Now that’ll be a very long pen. So that’s probably not
a very good estimate either. So it looks like our answer is B, twelve centimetres. So the answer then, the best estimate for a pen from that list is about twelve