15.
Here is a easy way to do it.
The first number's value is 1
The next number's value is 2
And so on you multiple the number before this one by 2
So the next value is 4, after that it's 8, next is 16. And all of this ends at 128 in computers storage and this is a 8 number string which we also call a byte. Now here is the actual trick. You would assign 0-128, 0-64, 1-32, 1-16, 1-8, 1-4, 1-2, 1-1. Now you just add the 1 values not the 0s and so if you add 1+2+4+8 you get 15.
