00:01
Okay, so the code here is three letters.
00:04
So our choice is 26 for each letter.
00:09
And here five digits, so zero to nine, so ten choices here.
00:16
And in part a, what we want is three letters, one, two, three, four, five numbers.
00:27
But we can't, we can repeat letters, but we can't repeat letters, but we can't repeat numbers and the first three entries must all be the same.
00:38
In other words, the first one here is a, a, a, or b, b, b, c, c, c, and so on.
00:46
So basically then what we have for these three letters are 26 choices, either a, a, a, b, b, c, c, c, c, up to z, z, z.
01:01
Now, here though, my first choice here, there are 10 choices and we have 9 i can't repeat and we have 8 only then 7 and then 6 choices so the number of code then will be this times this times this four times together and that i work out on a calculator so 26 times 10 times 9 8 times 7 and 6.
01:43
And the answer we get is 7 -8 -6 ,240 different codes possible in this case.
01:58
Okay, in part b, what we have is three letters, which must be from a, b, c or d...