Four kinds of question in rotation: simplifying a ratio to lowest terms, filling in an equivalent one, finding a unit rate, and solving a proportion. 64 : 80 reduces to 4 : 5; 135 points in 9 games is 15 points per game.
A ratio is a relationship between two numbers, and it stays a pair - simplifying 64 : 80 is one operation on the pair rather than two on its halves. That is what makes ratio different from the arithmetic around it, and why the colon is worth getting used to.
A comparison of two quantities, written with a colon. 4 : 5 says that for every four of the first thing there are five of the second. It says nothing about how many there are altogether, which is what makes it a ratio rather than a count.
So a ratio cannot be mistaken for a time. 12:18 looks like a clock reading, and this site prints those too, so ratios here are always written 12 : 18.
Mostly notation. 2 : 3 and 2/3 behave the same way when you scale them, which is why proportions are written as fractions and equivalent ratios with colons. These sheets ask both, deliberately - a student who can only do one has learnt a notation rather than an idea.
Sixth grade for ratios and unit rates, and sixth into seventh for proportions.