Hard Puzzles 

Workings

Hint
The first word is I.

Answer
I think, therefore I am.

A represents a, b, c, d and e. B represents f, g, h, i and j. C represents k, l, m, n and o. D represents p, q, r, s and t. E represents u, v, w, x, y and z. F represents z.

Workings

Hint
This is quite a common alphabet.

Answer
L.

The last letter from hoteL, which is H in the phonetic alphabet, alphA, bravO, charliE, etc.

Workings

Hint
One of answers involves royalty.

Answer
Prince, princes, princess.

Bra, bras, brass.

A similar change is care, cares, caress, but these are not nouns.

Workings

Hint
A tricky base 8 question.

Answer
61.

Each term is a square expressed in base eight, 7 2=49 = 61 in base 8.