Question

Consider strings of four decimal digits. Which rule must be used to find the number of strings of four decimal digits that have exactly three digits that are 9s? multiple choice the sum rule the subtraction rule the product rule the division rule How many strings of four decimal digits have exactly three digits that are 9s?

Answer 1

Answer:

36 possible ways

Step-by-step explanation:

Range of digit : 0 up to 9

To obtain the number of strings of 4 decimal digits that have exactly 3 digits that are 9s ; we use the multiplication rule :

In other to have exactly 3 digits from 4 that are 9s :

Say:

We have 3 9s and the last number could be any of the 10 possible digits except 9

First 9 = 1 possible way (since we have only one 9 between (0 to 9)

Second 9 = 1 possible way

Third 9 = 1 possible way

4th digit = 9 ways (could be any digit between 0 and 9, except 9)

Also, we consider the 4th digit's position ; as it could take up any of different positions in between the 9s = 4 ways

Using the product rule :

1 * 1 * 1 * 9 * 4 = 36 possible ways