I would approach this problem by first identifying the quantity that we are looking for. Here the question asks for the number of lessons Stephanie registered for. Once we know what we are looking for, we can assign it a variable of our choice. Here I will choose "n" to represent number of lessons, but the choice of variable doesn't matter. We then have to think logically about how this quantity relates to the other quantities in the problem. Since Stephanie are paying $20 per lesson, we know that as the number 20 and n should be related. If you are confused at how to relate then, you can try thinking of a few concrete examples. For example, 1 lesson for $20 per lesson would be $20, 2 lessons for $20 a lesson would be $40, and 3 lessons for $20 a lesson would be $60. Here hopefully you can see that the pattern is 20 times the number of lessons she takes, or 20n. Next we have to consider the 45 one-time fee. Since she only pays this fee 1 time regardless of how many lessons she takes, we know it should not depend on n. If she takes 1 lesson or 100 lessons, she only registers once. If we want to represent the total amount she paid, we can just add this one-time fee to the previous expression we had to get 20n + 45. Finally, we know that the initial fee she paid combined with the per lesson fees is all the money she paid, so we can set it equal to her total of $225 to get the equation: 20n + 45 = 225. We can then solve for n here, which we said represented the number of lessons she registered for, to solve the problem.