I think you're asking for how to write an equation to calculate total costs. If so, there are two types of costs here: fixed costs and variable costs. Since the coffee maker costs $70, that is your fixed cost - no matter how many servings you make, the coffee maker will always cost $70. The cost of the servings however is variable. This is because the total cost of the servings varies by how many times you make coffee. Thus, your equation written out would be something like this:
Total Coffee Cost = one-time payment of $70 for the coffee maker + $.20 each time you use the coffee maker
To put that in a more "math" format, you could write:
Total Coffee Cost = $70(1 time) + $.20 (?? times)
Since $70*1 will always equal $70, we can then take out the 1 to write:
Total Coffee Cost = $70 + $.20 (?? times)
Finally, instead of writing ?? times, we will use the variable "x" to represent this. And thus:
