T$_n$ = a + (n - 1)d
T$_4$ = a + 3d = 16 - - - - - - - - - - - - - - (i)
T$_8$ = a + 7d = 40 - - - - - - - - - - - - - - -(ii)
Solving simultaneously using the elimination method(subtract eqn (i) from eqn(ii)
4d = 24
d = $\;{24}/{4}$ = 6
Thus, the common difference = 6.
