To solve this question, we need to analyze the graph and determine the point where the two cell-phone plans have the same cost for a specific number of minutes used.

Step-by-Step Analysis

1. Identify the Plans:

   • Plan 1: This plan has a monthly fee of $9.99 and charges 5 cents (or $0.05) per minute for calls.
   • Plan 2: This plan has no monthly fee but charges 49 cents (or $0.49) per minute for calls.

2. Equation Setup for Each Plan:

   • Let $x$ represent the number of minutes used, and $y$ represent the total cost.
   • For Plan 1: $y = 9.99 + 0.05x$
   • For Plan 2: $y = 0.49x$

3. Finding the Intersection Point:

   • To find the number of minutes at which the two plans cost the same, we set the equations equal to each other: $9.99 + 0.05x = 0.49x$
   • Rearranging the terms: $9.99 = 0.44x$
   • Solving for $x$: $x = \frac{9.99}{0.44} \approx 22.7$

4. Approximating from the Graph:

   • On the graph, the intersection point appears to be closest to 21 minutes, as indicated by the marked answer "C" (21).

Answer

The approximate number of minutes Riley must use in a month for the cost of the two plans to be the same is 21 minutes.

Would you like more detailed steps or further questions? Here are some related questions that could expand your understanding:

1. How would the break-even point change if the monthly fee in Plan 1 increased?
2. What would happen to the break-even point if Plan 2 reduced its per-minute charge?
3. How can you derive the break-even point algebraically without using a graph?
4. What is the monthly cost for each plan if Riley uses 50 minutes in a month?
5. How does the cost difference between the two plans change as the number of minutes used increases?

Tip: Whenever comparing plans with a fixed fee and a variable rate, set up equations to find the break-even point where the costs are equal.