Two participants, Leo and Marco, competed for the same amount of time
During that time, Leo ate 36 hot dogs, and Marco ate 30 hot dogs.
We want to know,
Marco’s average rate, in hot dogs per hour
From the given info, lets say
Total time = t hrs
Leo's rate of eating hot dogs = 36/t
Marco's rate of eating hot dogs = 30/t
Given the statements,
(1) Marco’s average rate was 6 hot dogs per hour less than Leo’s.
=> 30/t = 36/t - 6
=> t = 1
=> Marco's rate of eating hot dogs = 30/t = 30/1 = 30 hotdogs/hr
Statement (1) is sufficient
(2) In the first 20 minutes, Leo ate 12 hot dogs.
This tells us something about Leo's rate in the first 20 minutes, BUT there are more things in the dark i.e, we don't know if Leo kept eating at that rate, it tells us nothing about Marco and we dont know the total time they ate together for
So it is not sufficient
Statement (2) is not sufficient
A. Statement (1) alone is sufficient, but (2) alone is not