Solving probability questions

[a157]

I need help with solving these combinatoric questions:

1. The student forgot the four-digit identification code of his credit card. What is the probability that a student will receive his money by typing a code at random if he remembers that:

a) all digits of the code are different;
b) the code does not contain the digits 0 and 1?
P.S. a and b are two separated cases

1. 15 parcels came to the orphanage. In four of them - winter things, in one - a leather jacket, in the rest - books. Workers open three parcels randomly. What is the probability that:

a) in two of them - winter clothes and in one - a leather jacket;
b) all three parcels - with books?
P.S. a and b are two separated cases
I tried solving them using formulas and logic
1.a) the total number of possible combinations is 10! / 6! = 5040, and the probability 1/5040?
b) there are only 8 possible digits, repetitions are possible, so the probability is 1/4096

1. a) 2/455

b) 24/91
I've done all calculations using fractions and multiplication because I don't know how to do these using formulas and which

 

[Cubist]

1. a) 2/455

I got the same answers as you EXCEPT for the one above.
You didn't show working for this answer. This makes it hard to give advice, but maybe you forgot that the leather jacket could be in the first, second or third parcel?

 

[a157]

I got the same answers as you EXCEPT for the one above.
You didn't show working for this answer. This makes it hard to give advice, but maybe you forgot that the leather jacket could be in the first, second or third parcel?

right, i forgot - it should be 6/455?