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