**This task is on the NRICH website and is relatively easy. The hardest part is that the relations between quantities are expressed as fractions or multiples of each other, so that quantities are worked out by scaling rather than by adding. It is an example of how a relatively simple context can be used to extend learners’ understanding of multiplicative relationships.**

**The questioning could be extended to include inverses such as one quantity being two-thirds of another means that the second is 3/2 times the first. The mixture of units, litres and millilitres, allows for the relation between them to be seen as also one of scaling. The context can be returned to again and again for more complex questions to be devised.**