You are correct...there are only 15 primary hydrogens. Sorry for the typo.
Here is the best way to think of chemical equivalence. Think of a fan. Now imagine painting one of the fan blades white. It doesn't matter which one you paint because all of the fan blades were identical to start with. Your friend could do the same thing on his fan, and without you telling him which blade to paint, would end up with exactly the same result.
This is the same concept in chemistry. Suppose I have ethene H2C=CH2. If I substitute any single hydrogen with another atom, I end up with a mono-substituted ethylene. But it doesn't matter which hydrogen atom I substitute because I get the same product in each case...1-substituted ethene. As a result, all four hydrogen atoms are chemically equivalent.
Now, back to your problem. Note that if you put a chlorine on ANY ONE of the atoms in Group C, you end up with 2-chloro-2,3,4-trimethylpentane. Note that "4-chloro-2,3,4-trimethylpentane" is exactly identical to 2-chloro-2,3,4,-trimethylpentane if you rotate the molecule along the long axis of your paper. This is why these positions of 2,3,4-trimethylpentane are said to be chemically equivalent. You can react at either position to give the same product.