For example, 

If I needed to generate a random number. I use a good source of entropy to generate `r`.

Then I add values `a`, `b` and `c` whom are generated from bad entropy such that `new entropy = r + a + b + c` 

How would the overall entropy look like?

The plus sign means to add and not concatenate. I believe that if we concatenated, then this would lead to an overall bad entropy because if we assume that each `a,b,c,r` each contribute 64 bits to 256 bits of entropy. We would in reality only have 64 bits of good entropy from `r`. This is my intuition with concatenation.