WAEC - Further Mathematics (2020 - No. 31)

A three-digit odd number less than 500 is to be formed from 1,2,3,4 and 5. If repetition of digits is allowed, in how many ways can this be done?
125
75
60
36

Explanation

To find the number of three-digit odd numbers less than 500 that can be formed using the digits 1, 2, 3, 4, and 5 with repetition allowed, we follow these steps:

 Identify the conditions:
Three-digit number: The number must have three digits.
Odd number: The last digit must be odd.
Less than 500: The first digit must be less than 5.

Choose the last digit:
The last digit (which determines if the number is odd) can be one of the odd digits: 1, 3, or 5. Thus, there are 
\(3 \text{ options for the last digit.}\)

Choose the first digit:
The first digit must be less than 5. Therefore, it can be one of the digits: 1, 2, 3, or 4. Thus, there are 
\(4 \text{ options for the first digit.}\)

Choose the middle digit.
The middle digit can be any of the digits: 1, 2, 3, 4, or 5 (since repetition is allowed). Thus, there are 
\(5 \text{ options for the middle digit.}\)

Calculate the total combinations:
Now we multiply the number of choices for each digit:
\(\text{Total combinations} = (\text{options for first digit}) \times (\text{options for middle digit}) \times (\text{options for last digit})\)
\(\text{Total combinations} = 4 \times 5 \times 3 = 60\)
The number of three-digit odd numbers less than 500 that can be formed is 60.

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