Adding and subtracting in two units

In the last lesson, you changed measurements in two units into the smaller unit, and back again. One litre is one thousand millilitres. A carton of milk holds about one litre, and one millilitre is about twenty drops. One kilogram is one thousand grams. A bag of sugar is about one kilogram, and a paper clip is about one gram. One kilometre is one thousand metres, about a fifteen-minute walk. One metre is about one big step. Today, we will change to one unit before we calculate, then give the unit the question asks for. Amara has one litre three hundred millilitres of orange juice in a jug. She pours in nine hundred millilitres more. How much juice is in the jug now? Give your answer in litres and millilitres. The jug has one litre three hundred millilitres of juice. Amara pours in nine hundred millilitres more. We must find the total, in litres and millilitres. The two amounts are in different units. So first, we change them to the same unit, millilitres. One litre three hundred millilitres is one thousand, three hundred millilitres. Now both parts are in millilitres. Three hundred and nine hundred make one thousand, two hundred. So one thousand, three hundred plus nine hundred is two thousand, two hundred. The question asks for litres and millilitres. So we change the total back. Two thousand, two hundred millilitres is two thousand millilitres plus two hundred millilitres. Two thousand millilitres make two litres, so that is two litres two hundred millilitres. There are two litres two hundred millilitres of juice in the jug now. We subtract the same way: one unit first.

A bag of rice has a mass of three kilograms. Amara's mother uses four hundred grams of the rice. What is the mass of the rice left? Give your answer in kilograms and grams. Try it first. Pause if you like. The bag has three kilograms of rice. Amara's mother uses four hundred grams. We must find the rice left, in kilograms and grams. First, three kilograms is three thousand grams. Three thousand minus four hundred is two thousand, six hundred. The question asks for kilograms and grams. Two thousand, six hundred grams is two thousand grams plus six hundred grams. That is two kilograms six hundred grams. The rice left has a mass of two kilograms six hundred grams.

Siti walks one kilometre two hundred metres to the park. Then she walks six hundred and fifty metres to the library. How far does she walk altogether? Give your answer in metres. Try it first. Pause if you like. Siti walks one kilometre two hundred metres to the park. Then she walks six hundred and fifty metres to the library. We must find the whole walk, in metres. One kilometre two hundred metres is one thousand, two hundred metres. One thousand, two hundred plus six hundred and fifty is one thousand, eight hundred and fifty. The question asks for metres, so we do not change it back. Siti walks one thousand, eight hundred and fifty metres altogether. Change to the same unit before you add or subtract. Then give the answer in the unit the question asks for.