Digit Arrangements
1. **State the problem:** We want to find the number of arrangements of the digits 2, 4, 5, 5, 6, 6, and 7 that form an even number greater than 6000000.
2. **Analyze the conditions:**
- The number must be 7 digits long using all digits exactly once.
- The number must be even, so the last digit must be even (2, 4, or 6).
- The number must be greater than 6000000, so the first digit must be 6 or 7.
3. **Digits available:** 2, 4, 5, 5, 6, 6, 7
4. **Case 1: First digit = 7**
- Since 7 > 6, any number starting with 7 is > 6000000.
- Last digit must be even: possible last digits are 2, 4, or 6.
- We have two 6's, so last digit can be 2, 4, or 6.
5. **Count arrangements for Case 1:**
- Fix first digit = 7.
- Fix last digit = one of {2, 4, 6}.
- Remaining digits to arrange in the middle: 5 digits from {2,4,5,5,6,6} excluding the last digit chosen.
For each last digit:
- If last digit = 2, remaining digits: 4, 5, 5, 6, 6
- If last digit = 4, remaining digits: 2, 5, 5, 6, 6
- If last digit = 6, remaining digits: 2, 4, 5, 5, 6 (one 6 removed)
Number of arrangements for 5 digits with duplicates:
- For digits with two 5's and two 6's: total permutations = \frac{5!}{2!2!} = \frac{120}{4} = 30
- For digits with two 5's and one 6: total permutations = \frac{5!}{2!} = \frac{120}{2} = 60
Calculate for each last digit:
- Last digit = 2: 30 ways
- Last digit = 4: 30 ways
- Last digit = 6: 60 ways
Total for Case 1 = 30 + 30 + 60 = 120 ways
6. **Case 2: First digit = 6**
- Number must be > 6000000, so first digit = 6 is allowed.
- Last digit must be even: 2, 4, or 6.
- Since one 6 is used as first digit, remaining digits are 2, 4, 5, 5, 6, 7.
For last digit:
- Last digit = 2, remaining digits for middle: 4, 5, 5, 6, 7
- Last digit = 4, remaining digits for middle: 2, 5, 5, 6, 7
- Last digit = 6, remaining digits for middle: 2, 4, 5, 5, 7
Number of arrangements for 5 digits with duplicates (two 5's):
- Total permutations = \frac{5!}{2!} = 60
For each last digit, 60 ways.
Total for Case 2 = 3 * 60 = 180 ways
7. **Total number of arrangements:**
$$120 + 180 = 300$$ ways
**Final answer:** 300 ways