Total in each case = 15.
and (6 + 8)/2 = 7
Therefore (8 + 4)/2 = 6.
and 1 + 3 + 5 + 8 = 17
Similarly, 1 + 4 + 6 + 9 = 20.
(2 x 5 - 4) = 6
(4 x 5 - 9) = 11.