Using the six digits 0, 1, 2, 3, 4, 5:
(1) How many four-digit even numbers with no repeated digits can be formed?
(2) How many five-digit numbers that are multiples of 5 with no repeated digits can be formed?
Problem 130
Official solution
(1) There are three cases to consider for forming four-digit even numbers:
- Case 1: The unit digit is 0. In this case, we have to choose three different digits for the thousands, hundreds, and tens places from the remaining five digits (1, 2, 3, 4, 5). This can be done in ways, since order matters and repetition is not allowed.
- Case 2: The unit digit is 2. The thousands place can be selected from the other four digits (1, 3, 4, 5) in ways. Then, for the hundreds and tens places, we choose from the remaining three digits in ways. Therefore, there are four-digit even numbers ending in 2.
- Case 3: The unit digit is 4. This case is similar to Case 2. The thousands place is chosen from the four digits (1, 3, 4, 5) in ways, and the hundreds and tens places are selected from the remaining three digits in ways. Again, we have numbers.
Adding up the numbers from all three cases, we have the total count of four-digit even numbers as:
Therefore, there are four-digit even numbers with no repeated digits.
(2) For forming five-digit numbers that are multiples of 5 and have no repeated digits, there are two cases:
- Case 1: The unit digit is 0. Since the number must be a multiple of 5, the tens digit is fixed as 0. The other four digits (1, 2, 3, 4, 5) can be arranged in the remaining four places in ways.
- Case 2: The unit digit is 5. Here, the thousands, hundreds, tens, and millions places can be chosen from the other four digits (0, 1, 2, 3, 4) in ways. We use for the first digit to ensure it's not 0.
Summing up the numbers from the two cases, we have the total count of five-digit numbers as:
Therefore, there are five-digit numbers that are multiples of 5 with no repeated digits.