Final answer:
The rate of children to balloons at a birthday party with a ratio of 12:17 can be expressed as '12 children for every 17 balloons', illustrating a proportional relationship.
Step-by-step explanation:
If a birthday party has 12 children and 17 balloons with a ratio of 12:17, the rate can be expressed as "12 children for every 17 balloons."
This is a way of expressing a proportional relationship between two quantities, where the first number in the ratio corresponds to the first quantity (children) and the second number corresponds to the second quantity (balloons).