If a set has N elements, it has 2N subsets.


 {a,b,c,d}


This set has 4 elements, so it has 24 or 16 subsets:

 1. { } or ⌀  <-- the empty set  
 2. {a}
 3. {b}
 4. {c}
 5. {d}
 6. {a,b}
 7. {a,c}
 8. {a,d}
 9. {b,c}
10. {b,d}
11. {c,d}
12. {a,b,c}
13. {a,b,d}
14. {a,c,d}
15. {b,c,d}
16. {a,b,c,d}  <-- the whole set, which IS considered a subset, but
                   it is not considered a PROPER subset, like the other 15.

Edwin