If I'm not wrong, during my education times usually such problem are solved using algebra. However during my younger sister education times, it seem to have changed. The teachers are teaching them a new way of tackling such problems. It involve drawing of diagram and tables and it is actually much faster and easier for kids to visualize compared to algebra.
I guess for primary kids, algebra are too deep for them. They should be using the diagram and tables they are taught to solve such problems.
Btw, I still dunno the new way of solving such problems. Me belong to the old school of teachings. It will be ideal more for kids to learn the diagram and tables now and then later when in secondary school learn the algebra method.
