A phone company charges $25 per month, plus an installation fee of $50. If you have paid the phone company $375, how many months have you had the phone?

The total bill was inclusive of the one-time payment of $50 and an additional payment of $25 per month. The only unknown variable here is the amount of months you've had the phone. This can be solved by isolating the unknown variable, like so:

Let: x = number of months

Total bill = one-time payment + monthly payment * number of months

$375 = $50 + $25x

375 - 50 = 25x

325 = 25x

325/25 = 25x/25

x = 13

Therefore, you've had the phone for 13 months.