You may be surprised at the answer: it isn't always 2.
In Boolean algebra, 1+1=1 because "+" means "OR"; if two statements are true then one or the other is also true.
In binary arithmetic, 1+1=10 because binary arithmetic uses only 0 and 1 to express all numbers and is the basis of computer logic. 10 means one 2 and no remainder, just as in decimal arithmetic 10 means one 10 and no remainder (no "ones").