A set of ordered pairs is a function when no input turns up twice with different outputs. These free printable worksheets ask that of a list of pairs, where the inputs are scattered among the outputs and have to be found first.
If they get stuck
The answer key prints on a separate page.
The pairs are written out of order on purpose. (1, 3) (4, 5) (1, 7) is not a function because the input 1 points to both 3 and 7 — but sorted, that clash sits in plain sight, and a child could answer by hunting for a repeat instead of reading the definition. Left scattered, the first numbers have to be picked out and compared, which is the actual skill.
Every sheet mixes in pairs that repeat an OUTPUT and are still functions — (2, 5) (4, 5) is fine, because two inputs may share a result. Roughly half the relations that repeat a number are functions, so duplicate-spotting fails and the rule has to be applied. This is the ordered-pairs half of the function question, kept separate from the table half because in a list the inputs are the harder thing to find.
One question from a sheet like this one, worked a step at a time — with a sentence saying what just happened and why.
Function from ordered pairs, step by step