Let no. of adult ticket sold = a
Let no. of child tickets sold = c

So, we are being asked if 12a + 8c > 20000, or in other words, is 3a + 2c > 5000

(1)
a + c = 2000
This doesn't say anything about the numbers sold. If we assume equal number of tickets for each were sold (i.e. 1000 each), then the 3a + 2c = 5000, whereas if number of adult tickets is greater than number of adult tickets (lets say a = 1500, c =500), then 3a + 2c = 5500

Not sufficient

(2)
a > c
Again, on its own it doesnt say much

Combine (1) + (2), and then we can say that the number of adult tickets was atleast 1001, and if we use that value, we can conclusively say that 3a + 2c > 5000

Hence, (D)

OA pls?