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.
