Rates and proportions
Find the value of one unit and then multiply, or cross-multiply the proportion and solve for the unknown.
A worked example
If 8 tickets cost $52, one ticket is 52 ÷ 8 = $6.50, so 14 tickets cost 14 x 6.50 = $91.
The mistake people actually make
Keep matching quantities in matching positions. Writing litres over kilometres on one side and kilometres over litres on the other flips the answer.