first your going to do the least common multiple which is 12 for both then your going to subtract because were trying to find out how many students did there work correctly. so, whatever you do to the denominator you do to the numerator so, its going to be 10/12 - 9/12= 1/12 is your answer.
Let the number of years after 2001 be x, then 1280000 + 19500x = 1700000 19500x = 1700000 - 1280000 = 420,000 x = 420000/19500 = 22 years. x = 22 years.
