graph word problems

Question

Andy and Devon are comparing the international calling plans on their cell phones. On his plan, Andy pays $3 just to place a call and $3 for each minute. When Devon makes an international call, she pays $5 to place the call and $2 for each minute. A call of a certain duration would cost exactly the same under both plans. What is the cost?

Answer 1

Shaya,

 

Let's break these down into 2 equations.  The cost of each call will be charge to place a call (in $) + the number of minutes times the cost per minute.

 

For Andy:

 

Cost = 3 + 3x  where x is the number of minutes.

 

For Devon:

 

Cost = 5 + 2x  where x is the number of minutes

 

The question is asking when do the cost equal each other.  To do this, set the equations equal to each other and solve for x:

 

3 + 3x = 5 + 2x

 

Subtract 3 from both sides to get:

3x = 2 + 2x

 

Subtract 2x from both sides to get:

 

 x = 2

 

So, the time when the cost are equal is 2 minutes.

 

Now, all we need to do is solve for the cost.  We can use either equation:

 

Cost = $3 + 3x = 3 + ($3/minute)(2 minutes) = $9

 

Cost is $9.

 

Hope this helps.