Ratios and dilutions
This task is on the NRICH website. It uses realistic data about dilutions to pose questions about adjusting proportions. You have mixtures of given proportions, and can tinker with them to add quantities and adjust to new dilutions, or to get back the original dilutions.
The context prevents any easy ad hoc methods and therefore helps towards an understanding of how to identify units which can be used as the basis for several cases. The numbers involved are realistic, and therefore do not offer exact integer arithmetic.
There are opportunities for learners to pose their own questions and an accompanying Flash interactive activity.