Random Puzzles 

Workings

Hint
The code has just four letters, A, B, C, and D. Each letter encodes 5 different letters.

Answer
I think, therefore I am.

Reasoning
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, and y.

Workings

Hint
There are 16 ounces in a pound, and 144 in a gross.

Answer
Seconds in an hour.

Reasoning
There are 16 x 144 = 2,304 ounces in gross pounds.

There are 60 x 60 = 3,600 seconds in an hour.

Workings

Hint
The first letters of one half of the words are: A, B, D, C, F, D, H, A, I.

Answer
ABUNDANT   + SCARCE
BRAVERY    + COWARDICE
DANGER     + SAFETY
CONTRACT   + EXPAND
FAMOUS     + UNKNOWN
DISTRIBUTE + GATHER
HORIZONTAL + VERTICAL
ACCIDENTAL + INTENTIONAL
IGNORANCE  + KNOWLEDGE

Workings

Hint
There are quite a few anagrams, one of which is LEAST.

Answer
Least, slate, stale, steal, tales, teals.

Remember that the question asked for common words.