Number of planes with maximum atoms Number of planes in $\text{SiH$_2$FCl}$ which contain maximum atoms but all must be different?


I am interested in the above problem. 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. But the book writes an answer $7$ to the above problem!! Any help will be highly appreciated.
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.