What is the missing letter in the following series Y E U I Q M _?

Answer 1

Y - 25
E - 5
U - 21 ===> Y-4 = 25 - 4 = 21
I - 9 ===> E + 4 = 5 + 4 = 9
Q - 17 ===> U - 4 = 21 - 4 = 17
M - 13 ===> I + 4 = 9 + 4 = 13
M - 13 ===> Q - 4 = 17 - 4 = 13

Y E U I Q M M