Per
Definition 1 of 3
Pro Tip 1/3
Quantifying Costs
Use 'per' to talk about prices in relation to a unit of measure. Gasoline is sold per litre.
Pro Tip 2/3
Rate Frequency
Use 'per' to specify the frequency of an occurrence within a time period. She goes to the gym five times per week.
Pro Tip 3/3
Statistical Figures
Use 'per' when providing statistical information, such as ratios or averages. There are 35 students per teacher in this school.