A, A, B, D, C, I, D, ?

How many 3 letter words can you make from the letters A,B,C and D, where letters cannot be repeated and are in alphabetical order?

How many 10 letter words can you make from the letters A,B,C,D,E,F,G and H?

If a, b, c, and d are in H.P., What can we say about the terms bcd, acd, abd, and abc

1, 3, 4, 8, 15, 27, (.....)(a) 37 (b) 44(c) 50 (d) 55

3. How many subsets of {a, b, c, d, e, f, h, i} are there? Show how you determined it.