if a is less than 80% of b.
a should be less than 0.8b. that means a can be almost 0.8b or can be 06.b or 0.4b or 0 (any value less than 0.8b)

considering option 1 alone..
lets take a = 0.8b
so b -a = 20 -> b - 0.8b = 0.2b = 20
b = 100
so, a = 0.8 * 100 = 80
a > 50 in this case.
a can also be zero. which, is a < 50.
so, we can`t answer whether a is greater than 50 or not.

considering option 2 alone..
when b is less than 50. any percent less than 100 of b should be less than 50. it is given a is less than 80% of b. so a is less than 50.

hence, it can be answered using b alone