Converting compound measurements

Let's remember the four pairs of units. One metre is one hundred centimetres. A big step is about one metre, and a fingertip is about one centimetre wide. One kilometre is one thousand metres. That is about a fifteen-minute walk. One kilogram is one thousand grams. A bag of sugar is about one kilogram, and a paper clip is about one gram. One litre is one thousand millilitres. A carton of milk holds about one litre, and one millilitre is about twenty drops. Today, we will change a measurement in two units into the smaller unit, and back again. A watermelon has a mass of two kilograms forty-five grams. What is its mass in grams? The watermelon has a mass of two kilograms forty-five grams. We must write its mass in grams only. One kilogram is one thousand grams. So each kilogram on the bar is one thousand grams. Two kilograms make two thousand grams. The forty-five grams are already grams. They stay forty-five grams. So two kilograms forty-five grams is two thousand grams plus forty-five grams. Two thousand plus forty-five is two thousand and forty-five. The watermelon has a mass of two thousand and forty-five grams. Metres are different: one metre is only one hundred centimetres. We can also change back to two units.

A skipping rope is three hundred and five centimetres long. What is its length in metres and centimetres? Try it first. Pause if you like. The rope is three hundred and five centimetres long. One metre is one hundred centimetres. Mark off one metre, two metres, three metres. Three hundred centimetres make three metres. Five centimetres are left over. They stay as they are. So three hundred and five centimetres is three hundred centimetres plus five centimetres. That is three metres five centimetres. The skipping rope is three metres five centimetres long.

The path around a pond is two thousand, six hundred metres long. What is its length in kilometres and metres? Try it first. Pause if you like. The path is two thousand, six hundred metres long. One kilometre is one thousand metres. Mark off one kilometre, two kilometres. Two thousand metres make two kilometres. Six hundred metres are left over. They stay as they are. So two thousand, six hundred metres is two thousand metres plus six hundred metres. That is two kilometres six hundred metres. The path is two kilometres six hundred metres long. Change the whole big units first: one hundred for metres, one thousand for the others. The extra small units stay as they are.