Pick up a ball and put it back. Pick a ball at random, there are 16 situations: (1, 1), (1, 2), (1, 3), (1, 4), (.

Among them, there are four cases with the same number twice: (1, 1), (2,2), (3,3) and (4,4).

So p (same tag twice) =4/ 16= 1/4.

2 The sum of two extracted labels is 4.

There are four situations in which the sum of the numbers extracted twice is 4: (1, 3), (2,2), (3, 1).

Probability =3/ 16.