Final answer:
The question asks for the sum of A, B, C, and D where each represents a different digit. However, without specific values or relationships given for these letters, it's not possible to provide an answer to the question as presented. The references provided discuss significant digits, a separate topic that doesn't offer enough to solve the original query.
Step-by-step explanation:
The question presents a scenario where each letter represents a different digit. The goal is to find the sum of A + B + C + D. Based on the mention of digits and numbers, it's clear that this is a math problem related to basic arithmetic and potentially the properties of numbers.
However, the information provided does not include any specific values or relationships between A, B, C, and D that would allow us to determine their sum. Thus, without additional context or constraints, it's impossible to solve the problem as stated. If the question intended to ask about significant digits based on the snippets provided, then that's a separate topic which deals with the precision of numbers in mathematical operations.
Based on the topic of significant digits given in the reference information, when adding or subtracting numbers, normally trailing zeros, which are not significant, are dropped unless they are necessary to indicate the precision of the calculation. However, this principle of significant digits does not directly apply to the question of determining the sum of A, B, C, and D where each represents a different digit without additional context.