Average of all even numbers between 104 and 148 is.

$$\eqalign{ & {\text{Average}} = \frac{{{\text{First number}} + {\text{Last number}}}}{2} \cr & {\text{So,}} \cr & {\text{First number}} = 104 \cr & {\text{Last number}} = 148 \cr & {\text{Average}} = \frac{{252}}{2} = 126 \cr} $$

Alternate:
Number of term between 104 and 148
146 = 106 + (n - 1)2
n = 21
Then, addition of all 21 term
$$\eqalign{ & = \frac{{21}}{2}\left[ {2 \times 106 + \left( {21 - 1} \right)2} \right] \cr & = \frac{{21}}{2}\left[ {212 + 40} \right] \cr & = \frac{{21}}{2} \times 252 \cr & {\text{Average}} = \frac{{21}}{2} \times \frac{{252}}{{21}} = 126 \cr} $$