Data Encryption Standard (DES): Can someone explain this in more detail please:

Calculate the 20th and 3rd output bits for input string and round key: X;1X; = 1100|1100110011001100 ... |1100 1100 16 times K=1001|100110011001|1001 .-- |1001 1001 12 times X12, ..., X17 = 011001 = y19, ..., y24, k19, ..., k24 = 011001. Y19, ..., 724 + k19, ..., k24 = 011001+011001 = 000000. 54(000,000) = 7= 0111 213, 214, 215, 216 = P:1= 214 H 120 = 1 So the 20th bit of F(ki,X;) is 1. Recall that Xi+1 = X;-1 + F(ki, X;) so we must add the 20th bit of X:-1 = 11001100110011001100110011001100. The 20th bit of X;1 is 0, so the the 20th bit of Xi+1 is 1