mrprajesh wrote:I eliminated a and c as PK Csystem has 2 key [public and private] so 17*2=34. it was told d is answer [nC2] but could not understand the theory behind it, any thoughts?

Public key system:

Every person has 2 keys

17 persons so total = 34 keys

Secret key system:

Here every pair of person will have a common key

eg lets say 3 persons a, b, c

[a, b] will have common key

[a, c] will have common key

[b, c] will have common key

therefore find distinct pairs from a set of 17

therefore 17C2 = 136

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