Well, to build 128 x 1 MUX,
First, use 32 number of 4 x 1 MUX , That will result in 128 input lines and 32 output lines. This is your “X”.
Second, use 2 number of 16 x 1 MUX, That will result in 32 input lines and 2 output lines . This is your “Z”. connect these 32 input
lines to “X” output 32 lines.
Third, use 1 number of 2 x 1 MUX, That will result in 2 input lines and 1 output line. This is your “Y” . connect these 2 input lines to
“Z” output lines.
so , (X+Y+Z) = 32 + 1 + 2 = 35.