Math Problem Statement

A phone company offers two monthly plans. Plan A costs

US$22

plus an additional

US$0,11

for each minute of calls. Plan B costs

US$14

plus an additional

US$0,15

for each minute of calls.

For what amount of calling do the two plans cost the same?

What is the cost when the two plans cost the same?

Solution

Let's define the number of minutes of calls as xx.

Plan A:

  • Fixed cost: $22
  • Variable cost: $0.11 per minute

The total cost for Plan A can be expressed as: Cost of Plan A=22+0.11x\text{Cost of Plan A} = 22 + 0.11x

Plan B:

  • Fixed cost: $14
  • Variable cost: $0.15 per minute

The total cost for Plan B can be expressed as: Cost of Plan B=14+0.15x\text{Cost of Plan B} = 14 + 0.15x

Finding the amount of calling where the two plans cost the same:

To find the number of minutes where both plans cost the same, we set the two cost expressions equal to each other and solve for xx:

22+0.11x=14+0.15x22 + 0.11x = 14 + 0.15x

Subtract 14 from both sides: 8+0.11x=0.15x8 + 0.11x = 0.15x

Subtract 0.11x0.11x from both sides: 8=0.04x8 = 0.04x

Solve for xx by dividing both sides by 0.04: x=80.04=200x = \frac{8}{0.04} = 200

So, the two plans cost the same when you make 200 minutes of calls.

Cost when the two plans cost the same:

To find the cost, substitute x=200x = 200 into either cost equation (both should give the same result):

Using Plan A's cost equation: Cost of Plan A=22+0.11(200)=22+22=44\text{Cost of Plan A} = 22 + 0.11(200) = 22 + 22 = 44

So, when the two plans cost the same, the cost is $44.


Would you like more details on this solution or have any other questions? Here are some related questions to explore:

  1. How would the costs change if Plan B had a lower per-minute rate?
  2. What if Plan A had a lower fixed cost?
  3. How many minutes should one talk to make Plan A cheaper than Plan B?
  4. How do the costs differ if the number of minutes is 100?
  5. How would you graph the cost of both plans?
  6. What if a third plan was introduced with a different cost structure?
  7. How does a change in fixed cost affect the break-even point?
  8. How would rounding of costs to the nearest cent affect the results?

Tip: When comparing costs in similar scenarios, it's often useful to express both cost structures algebraically and find the intersection point for a clear comparison.

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Math Problem Analysis

Mathematical Concepts

Linear equations
Cost analysis

Formulas

Equation of a line

Theorems

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Suitable Grade Level

Grades 7-9