5

3!

(No, really, that's the right number. 3! = 3 * 2 * 1 = 6.)

7

Eight.

Nine

10

Pair of ones.

A dozen

13

Fourteen

15

Sixteen

17

Eighteen.

19

Twenty.

21

2+2*2+2*2*2*2 = 22

23

4!

(that is, 4*3*2*1=24)

25

26

27

Twenty-eight. :D

29

Thirty.

31

32

Pair of threes.

34

Thirty-five.

36

37

38

39

40

41

42

43

44