Maths Olympiad Prep

Track / Stage 3 / 130 of 260 #130 of 1964

Problem 130

AMC 10/12, early questions
Combinatorics Difficulty 3.3 Find the answer

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?

A number or a short expression. Spacing and $ signs are ignored.

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 A53A_5^3 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 A41A_4^1 ways. Then, for the hundreds and tens places, we choose from the remaining three digits in A42A_4^2 ways. Therefore, there are A41×A42A_4^1 \times A_4^2 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 A41A_4^1 ways, and the hundreds and tens places are selected from the remaining three digits in A42A_4^2 ways. Again, we have A41×A42A_4^1 \times A_4^2 numbers.

Adding up the numbers from all three cases, we have the total count of four-digit even numbers as:
A53+A41×A42+A41×A42=60+48+48=156. A_5^3 + A_4^1 \times A_4^2 + A_4^1 \times A_4^2 = 60 + 48 + 48 = 156.
Therefore, there are 156\boxed{156} 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 A54A_5^4 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 A41×A43A_4^1 \times A_4^3 ways. We use A41A_4^1 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:
A54+A41×A43=120+96=216. A_5^4 + A_4^1 \times A_4^3 = 120 + 96 = 216.
Therefore, there are 216\boxed{216} five-digit numbers that are multiples of 5 with no repeated digits.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.