base-2 binary;

The following numbers are written using the base-2 system:

21:

21 = 20 + 1

21 = 16 + 4 + 1

21 = (2^4) + (2^2) + (2^0)

21 = 10101

24:

24 = 16 + 8

24 = (2^4) + (2^3)

24 = 11000

30:

30 = 16 + 8 + 4 + 2

30 = (2^4) + (2^3) + (2^2) + (2^1)

30 = 11110