How can you recognize whether a real-world problem should be represented by an equation or inequality?

An equation:

It is used to express exact values.

Therefore, we will use an equation when when know that the quantity given is strictly equal to a given value.

Examples:

1 - Price of a kilo of apples is $2

This is translated as ... > cost of 1 kilo = $2

2 - The length of a ruler is 30 cm

This is translated as ... > length = 30 cm

An inequality:

It is used to show relation or compare between a quantity and a value. Therefore, we use it when we find keywords as "at least, at most, greater than, less than, ... etc"

Example:

1 - The distance between two cities is at least 20 miles.

This means that the distance can be 20 miles or more

This is translated as:

distance ≥ 20 miles