a=1, b=2, c=3,... z=26.
The number in the middle of each triple is the same as the digits of either end's number when added together.
3+7=10 = 8+2 and so on.
eg: first row ai = 19, dg = 46
now add 1+9 or 4+6 both are equal to 10, which was put in the middle.