F.5permutation and combination

1)
When a task in a computer game is completed,a code of 5 different digits will be generated at randoom.Find the total number of codes can be generated.
ans:30 240

2)By using 10 cards marked 0 to 9 respectively,
find how many 5-digit numbers that are not divisible by 10
can be formed?
ans:24 192

3) 6 different letters are taken from the word "BISECTOR" and rearranged.
Find the number of permutations which satisfy each othe following
reuirements.
a)the first letter is S or T.
b)the first and second letters are neither I or E.
ans: a) 5 040 b)10 800

4) Mr Chan,Mr Lee,Mr Cheung,Mr Wong,Mr Ho and Mr Mok are going to stand in a row for an opening ceremony.If Mr Chan stands at at the left-most or the right-most of the row,in how many ways can they be lined up>
ans: 240

pls show clear steps~thanks!