Choice Choice A is the correct answer. We model this with $20x + 50 = 170$, where $x$ is months.

1. Set up equation: $20x + 50 = 170$
2. Subtract 50 from both sides: $20x = 170 - 50$
3. Simplify: $20x = 120$
4. Divide both sides by 20: $x = \frac{120}{20}$
5. Simplify: $x = 6$
6. Verify: $20(6) + 50 = 120 + 50 = 170$ ✓

So, you have been a member for 6 months.

Choice Choice B is incorrect because this results from adding 50 to 170: $220$, then $220/20 = 11$. You subtract the joining fee.

Choice Choice C is incorrect because this is the cost of the monthly fees ($120), not the number of months.

Choice Choice D is incorrect because this results from dividing the total by the rate: $170/20 = 8.5$. This ignores the joining fee.