Two units in one question

Mei Ling has a roll of ribbon 4 metres long. She cuts off 8 equal pieces, each 35 centimetres long. What is the length of the ribbon left on the roll? Give your answer in metres. Try it first. Pause if you like. Here's the roll, end to end. 4 metres of ribbon. She cuts 8 pieces off this end, and every piece is the same. What's left is the purple part, and that's the length to find. So how much ribbon came off? 8 pieces. 8 times 35. two hundred and eighty centimetres came off the roll. Now look at the strip. There are two units on it. The roll is 4 metres. The cut is two hundred and eighty centimetres. Those two numbers are not the same kind of thing yet, so nothing can be subtracted. There are one hundred centimetres in a metre, so divide by one hundred. two hundred and eighty centimetres is two point eight metres. Now both numbers are metres. 4 metres of ribbon, and two point eight of it has gone. 4 minus two point eight. one point two. There's one point two metres of ribbon left on the roll. One strip, and two units written on it. Metres above, centimetres below. The middle line is the one that makes them the same. So change the units first, and then subtract.