nCr = n!/[r!(n - r)!]

20C2 = 20!/[2!(20 - 2)!] = 190 different combinations.. answer//

20 choices for the first but only 19 for the second; 20 • 19 = 380;

but choice AB is the same combination as choice BA; each combination appears twice so 380 ÷ 2 = 190 actual different combinations.

20C2 = 190