Step-by-step explanation:
The ratio of the dry cleaning fee to the number of sweaters is:

Because the dry cleaning fee is 18 for 3 sweaters.
Then, to calculate what is the cost to clean 1 sweater, we need to divide $18 by 3, as:

So, the cost to clean one sweater is $6
Therefore, the ratio that represents that is:

Now, the ratio 18:3 and 6:1 are equivalents, because we can divide the first ratio by 3, and the result is the second:

Answers: 1. 18:3
2. $6
3. 6:1 it is equivalent
4. 18/3 : 3/3 => 6:1