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

Hope this helps :)

Well, I guess you can tell by looking for some key words.

You use an equation when the situation has words like
-- is
-- equals
-- same as
-- results in .

You use an inequality when the situation involves words like
-- more
-- most
-- less
-- least
-- at least
-- no more than .