Answer :
First sequence with common difference:
8-x=y-8
Second sequence:
20-y=y-x
Solving two simultaneous equations:
x=16-y
20-y=y-16+y
y=12
x=4
8-x=y-8
Second sequence:
20-y=y-x
Solving two simultaneous equations:
x=16-y
20-y=y-16+y
y=12
x=4