Multiplicative Change and Proportion
Direct proportion means when one variable changes, the other changes at the same rate. Think currency conversion - if £1 = 90 rupees, then £10 = 900 rupees (multiply both sides by 10).
Conversion graphs always create straight lines because the relationship stays constant. To use them, find your starting point on one axis, draw a line to the graph, then across to the other axis. A ruler helps with accuracy.
Scale diagrams work similarly - if a car model has a 1:30 scale and the model is 10cm long, the real car is 300cm long. Maps use the same principle, though you'll often need to convert units first.
Pro Tip: When stuck with proportions, work out what one unit is worth first - it makes everything easier!