The average of the two-digit numbers, which remain the same when the digits interchange their positions, is__________?

A. 33
B. 44
C. 55
D. 66

Explanation:

Average = (11 + 22 + 33 + 44 + 55 + 66 + 77 + 88 + 99)/9

= [(11 + 99) + (22 + 88) + (33 + 77) + (44 + 66) + 55]/9
= [(4 * 110) + 55]/9 = 495/9 = 55