Number of planes with maximum atoms
I predicted the answer to 10 which indeed is $5\choose 3$ because maximum number of points in a plane will be 3 since the structure is tetrahedral. 


Yes, 10 is the number of sets with three atoms, and each set defines a unique plane.


  But the book writes an answer 7 to the above problem


3 of those (HHSi, HHF, HHCl) don’t count, so 7.