Can someone please help me with this question? Thank you!

Mort is to pay off a loan of$70,000 with equal payments at the end of every month over 10 years (i.e., 120 months). The ANNUAL effective rate is 8%. Mort decides that he can actually manage to pay double the monthly payment each month. How many MONTHS will it take him to pay off the loan? (Include the final month where the last payment will be smaller than all the rest.)