Explanation:
For each row in the picture, you have to switch on some of the bits to create the number on the left.
Each bit has a value, a "weight" so to speak. By switching on the bit, you add that weight to the sum.
These "weights" are shown at the top row.
So to create 4, it's easy, you just switch on the '4' bit.
Same for 128, only one bit has to be switched on.
40 is a bit more involved, now you have to switch on 32 and 8, since 32+8 is 40.
For every number, there is exactly one combination of switched on bits to create that number. In the case of 40, the binary representation is:
00101000.
