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?

Result

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

Solution:

Solution in text a =
Solution in text b =
Solution in text c =
Solution in text d =

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