A maths question for all out there

SPRINT has an initial month charge of $25.75 for providing their services and charges 30 cents for every minute.
VERIZON charges $55.50 per month and 10c for every minute.
CINGULAR does not have an initial monthly charge. It charges 75cents for every minute.
Note: No more free minutes included in plans!
How do you write the equation that represents each of the options. Use C to represent the cost and M to represent the minutes.
2. Which company would you choose if you talked an average of 100 minutes per month? I need to show the working.
3. by writing an equation and solving it, determine when SPRINT and VERIZON charge the same rate?

Now let's take the Total cost be C,
Time period be M,

SPRINT;
C=25.75+0.30M

VERIZON;
C=55.50+0.10M

CINGULAR;
C=0+0.75M

These are the equations for your problem.

2. M=100

Then :- SPRINT :- 25.75+100*0.30
=25.75+30
=55.75

VERIZON:- 55.50+100*0.10
=55.50+10
=65.50

CINGULAR:- 0+0.75*100
=75
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I would prefer SPRINT as it would be cheaper and efficient for 100 M.
But if you are a person who would speak for less than 35 minutes,Then go for CINGULAR.
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3. C=25.75+0.30*M ----- 1
C=55.50+0.10*M ----- 2

Equating 1 and 2

25.75 + 0.3 M = 55.50 + 0.1 M
55.50 - 25.75 = 0.2 M
M = 148.75

In this case both the cost will be the same.
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Please state whether my answer was useful.

1. Formula:
C of Sprint = 25.75 + 0.30(M)
C of Verizon = 55.50 + 0.10(M)