Help permutations and combination

[Robertwellson]

I can't solve number 4) a, the answer is 180 can you explain it aswell?

 

[Dr.Peterson]

View attachment 20173

I can't solve number 4) a, the answer is 180 can you explain it aswell?

The first digit of a 3-digit number can't be 0, but the rest can.

There are 6 possible first digits; then 6 for the second (any but whatever was used in the first), and then 5 for the third. So 6*6*5 = 180.

Now show your attempt for (b).

 

[Robertwellson]

The first digit of a 3-digit number can't be 0, but the rest can.

There are 6 possible first digits; then 6 for the second (any but whatever was used in the first), and then 5 for the third. So 6*6*5 = 180.

Now show your attempt for (b).

Why can't I use 0?

 

[Robertwellson]

Why can't I use 0?

Oh I understand it now, thank you for your help, we can't use 0 as the first number because for example 013 is technically 13. For b is 2*6P2 =60

 

[Dr.Peterson]

Oh I understand it now, thank you for your help, we can't use 0 as the first number because for example 013 is technically 13. For b is 2*6P2 =60

Correct on both. For (b), we could also say it is 2*6*5.