23

So, starting with 2⁴ :

Power of 2	Include?	Subtraction	Binary so Far
24 == 16	16 is less than 23	23 - 16 == 7	1 _ _ _ _
23 == 8	8 is greater than 23	not done	1 0 _ _ _
22 == 4	4 is less than 7	7 - 4 == 3	1 0 1 _ _
21 == 2	2 is less than 3	3 - 2 == 1	1 0 1 1 _
20 == 1	1 equals 1	1 - 1 == 0	1 0 1 1 1

So 23 represented in binary is 10111.

Check this by adding up the included powers of two:

16 + 4 + 2 + 1 = 23