Math logic

There are 20 children in the group, each two children have a different name. Alena and John are among them. How many ways can we choose 8 children to be among the selected

A) was John
B) was John and Alena
C) at least one was Alena, John
D) maximum one was Alena, John?

Correct result:

a =  50388
b =  18564
c =  75582
d =  196035840

Solution:

p1=7!=5040 a=1 19 18 17 16 15 14 13/p1=1 19 18 17 16 15 14 13/5040=50388
p2=6 5 4 3 2 1=720 b=1 1 18 17 16 15 14 13/p2=1 1 18 17 16 15 14 13/720=18564
p3=7 6 5 4 3 2 1 2=10080 c=3 19 18 17 16 15 14 13/p3=3 19 18 17 16 15 14 13/10080=75582
d=2 17 16 15 14 13 12 11=196035840=1.960358108



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