Permutaions&combinations4

(1)

He has 10 game discs, where 6 of which are tactical games and 4 are action games. He has a disc rack which can hold 6 game discs.

If he selects 4 tactical game discs and 2 action game discs out of 10 game discs to be put on the rack, how many permutations are there?

(2)

There are 7 numbers, 0,1,2,3,4,5 and 6. 4 different numbers are selected to form a 3-digit or 4-digit number. (When the thousands digit is 0, the number formed is treated as a 3-digit number.)

(a) How mant 3-digit numbers can be formed?

(b) How mant 4-digit numbers can be formed?