Tn_{n}n = a + (n - 1)d
T4_{4}4 = a + 3d = 16 - - - - - - - - - - - - - - (i)
T8_{8}8 = a + 7d = 40 - - - - - - - - - - - - - - -(ii)
Solving simultaneously using the elimination method(subtract eqn (i) from eqn(ii)
4d = 24
d = 244\frac{24}{4}424 = 6
Thus, the common difference = 6.
