PASCAL 2018 Problem 16

Reading from left to right, a sequence consists of 6 \mathrm{X} 's, followed by 24 \mathrm{Y} 's, followed by 96 \mathrm{X} 's. After the first n letters, reading from left to right, one letter has occurred twice as many times as the other letter. The sum of the four possible values of n is

Answer Choices
A. 72
B. 54
C. 135
D. 81
E. 111