There are two ways to read this:
Way 1: I gave 1/3 of a whole candy bar to one friend and 3/4 of the whole candy bar to another friend.
1/3 + 3/4 = 4/12 + 9/12 = 13/12 = 1 1/12
This can't happen because you started with exactly 1 original candy bar. You can't give away 1 1/12 candy bar if you only have 1 candy bar.
Way 2: I gave 1/3 of a candy bar to one friend and 3/4 of the remaining candy bar to another friend.
You gave 1/3 of a whole candy bar to one friend. That means you have 1 - 1/3 = 2/3 of a whole candy bar left.
You then gave 3/4 of the remaining candy bar to another friend. That means you have 1 - 3/4 = 1/4 of the remaining candy bar left.
But the remaining candy bar is 2/3 of a whole candy bar, so you have 1/4 left of 2/3 of a whole candy bar. In other words you have 1/4 * 2/3 = 1/6 of a whole candy bar left.