i) If there were no restrictions, the number of possible PIN codes would be .
The invalid codes come in 3 forms.
Codes like xxxy: There are 10 ways to choose x and then there are 9 ways to choose y, so there are possibilities.
Codes like yxxx: Similarly, there are possibilities.
Codes like xxxx: There are 10 of these.
So the number of allowable codes is .
Maybe you can work out ii) from this.