Q:

Probability of number sequence

A number is consist of 4 sequence numbers, 0 or 1.  First, we choose 4 numbers to make a number.  Next, we choose a number which differ in exactly two digits for the first one.  How many numbers can we take?
The answer is $2^4$${4 \choose 2}$.  How do you get ${4 \choose 2}$?

A:

The question is very unclear, but $4 \choose 2$ has to come from selecting which digits differ from the original number.

