This is an alternating addition and subtraction series.
In the first pattern, 10 is subtracted from each number to arrive at the next.
In the second, 5 is added to each number to arrive at the next.
First pattern (80) (80 - 10 = 70) (70 - 10 = 60) (60 - 10 = 50)
Second pattern (10) (10 + 5 = 15) (15 + 5 = 20)
So the answer is a = 20.