Q-stem rephrased- Is AB+AC+BC >20

St(1):- BC-AC=10  
=> BC = AC + 10

Since one of the sides is greater than 10, sum of the other sides is also greater than 10 and the perimeter greater than 20.This is because the sum of two sides is greater than the third side. (Sufficient)

St(2)Area of the triangle is 20  

For a given area  equilateral triangle has the smallest perimeter

Now if the triangle was equilateral with area 20 then its side would have been 20/3  and thus its area would have been √3/4 * (20/3) * (20/3) 

√3 * (100/9) which is 

  √3 * 11.11 and this is LESS than 20

 = >Thus p=20 is NOT enough to produce area=20 even for the best case (for equilateral triangle that has minimum perimeter) 
=>The Perimeter must be more than 20.  (Sufficient)

The second statement was indeed testing on the Maximum/Minimization model and first statement was testing the basic properties of triangles.