Yes, thanks everyone. I think the thing DS and I found tricky was things like writing 4,500,000mm in km. Do you have to change the mm into m first before you can then convert into km? Or is there another, quicker, way of doing it?
Another one was 250mg into kg.
The safest way is probably to convert first from mm to m and then m to km. Its quite quick because you just divide by 1000 to get the metres and then divide by 1000 again to get the km.
i.e. 4,500,000mm = 4,500 m = 4.5km.
Simliarly 250mg = 0.25g = 0.00025kg.
Its also worth them bearing in mind what the different units actually represent in real life. i.e. a km is a long distance, while a mm is a short distance. So there will be lots and lots of little mm in a single km. This allows you to check that the final answer looks reasonable. If it doesn't you've probably multiplied when you shoulkd have divided or vice versa.