Binary:
(from right to left)
First digit * 1
Second digit * 2
Third digit * 4
100 = 1*4 + 0*2 + 0*1 = 4 + 0 + 0 = 4 in base 10
101 = 1*4 + 0*2 + 1*1 = 4 + 0 + 1 = 5 in base 10
11 = 1*2 + 1*1 = 2 + 1 = 3 in base 10
110 = 1*4 + 1*2 + 0*1 = 4 + 2 + 0 = 6 in base 10