5. Petar has forgotten the PIN of his mobile phone. He remembers that it is a four-digit number that in its notation contains only the digits 3 and 4. What four-digit numbers could be Petar's mobile phone PIN? How many such numbers are there?

Each task is scored out of 10 points.

The use of a pocket calculator or any manuals is not allowed.
5. In a four-digit number, the following can appear:

2 digits 4 and 2 digits 3 or

3 digits 4 and one digit 3 or

3 digits 3 and one digit 4.

2 POINTS

Possible numbers are:

3344, 3443, 3434, 4433, 4343, 4334 POINTS

4443, 4344, 4434, $3444 \quad 2$ POINTS

3334, 3433, 3343, 4333 POINTS

There are 14 such numbers. 1 POINT

TOTAL 10 POINTS

Note: By listing all 14 numbers and correctly counting them, 10 points are obtained.