Final answer:
A two-character code system can be created for 3,000 survey recipients; one character for income bracket with 7 possibilities (A, 2-5, Y, Z) and one for voting intention (y, N, U), resulting in a total combination of 378 unique codes, which satisfies the requirement.
Step-by-step explanation:
To design a simple coding system that accommodates all 3,000 survey recipients with the criteria specified, we can create a system using a two-character code.
The first character indicates the income level with 7 different options. We can use digits 2-5 and letters Y and Z (6 possibilities), and choose one additional letter from A to X, let's say A to signify the income level, fulfilling the requirement of at least one letter and indicating the income bracket. Therefore, we have A, 2, 3, 4, 5, Y, Z depicting the income levels.
The second character indicates the intention to vote, where Y stands for yes, N stands for no, and U stands for undecided. Since the alphabet from Y to Z is case-sensitive, we can use lowercase 'y' to denote 'yes' and uppercase 'Y' to differentiate the income level.
Thus, we use a combination of an income level character (A, 2, 3, 4, 5, Y, Z) and a voting intention character (y, N, U) to produce a unique code. This provides us with 6*3=18 unique codes for the voting intention and 21*6*3=378 unique codes for the income levels. Since we only need 3000 codes, and we have 378 unique codes available, a two-character code suffices, making it the system with the fewest possible characters.