C(x) = 6x + 3 (in USD)
<h3>
Further explanation</h3>
<u>Given:</u>
- The fee to rent is $3 for the day.
- Gillian rented skates and skated for 3 hours and was charged $21.
<u>Question:</u>
The equation that represents cost, C(x), ice skating as a function of x, the number of hours of skating.
<u>The Process:</u>
We try solving a word problem about the single variable equation.
Step-1: calculate the cost of skates per hour.
Let the rate of using the skating rink be R per hour. We will seek the value of R.
On that day Gillian rented skating for 3 hours and paid $ 21. The equation that will determine this given by:

Both sides subtracted by 3.

We isolate R on the left side. Both sides are divided by 3.
Hence, the fee of skates per hour is 
Step-2: determine the cost equation for ice skating
Let x as the number of hours of skating.
Recall the fee of skates per hour is $6 and the fee to rent is $3 for the day.
Thus, the equation which represents the cost of ice skating as a function of x, the number of hours of skating, will be given by:
(in USD)
<h3>Learn more</h3>
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Keywords: the ice skating rink, charges an hourly fee, Gillian rented skates, the equation represents the cost, C(x), a function of x, the number of hours of skating, a word problem, the single variable equation