This answer is not correct. Remember that there is very little carbon-13 present because the isotope that is more abundant will have a mass closer to the average mass. You also did not quite remember the mathematical formula that is used to calculate the abundance. Use the expression...12X + 13(1-X) = 12.01115 and solve for X. You probably just added 12X + 13X and then divided 12.01115 by 25 to get your answer.