00:01
So in this case, we're talking about strings of decimal digits of length five.
00:05
So a good example, they give us a 0 -0 -312.
00:08
So it can start with zero.
00:10
We can obviously have 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.
00:16
So this is 10 options that we have.
00:19
So from that, let's first discuss how many strings in a begin with 774.
00:28
So if we're talking about it begins with 774, we only need to worry about the last two digits.
00:36
So since each digit has 10 choices, we're going to have 10 choices for one digit times 10 choices for the other digit.
00:42
So that means there's 100 strings.
00:45
Part b.
00:46
We know that we want to know how many strings in a have exactly 1 .8.
00:51
So that means that there can be 1 8 in any of the five places.
00:56
So that means that there's the four other places and there would be nine to the fourth choices...