In the sequence C U B A E D E D A B E B A U C D B C A D B D U B C A C B E D A, all the letters A are removed. In the resulting sequence without any A, which letter will be the eleventh letter from the left end?

Introduction / Context:
This question checks your ability to filter a sequence based on a condition and then perform position counting in the resulting sequence. The original sequence contains many letters, including several As, and you must mentally remove all As and then identify which letter appears at a specified position from the left.

Given Data / Assumptions:
• Original sequence: C U B A E D E D A B E B A U C D B C A D B D U B C A C B E D A.
• Every A in the sequence must be removed completely.
• After removing all As, we must find the eleventh letter from the left within the shortened sequence.

Concept / Approach:
This is a simple filtering followed by indexing problem. The process is to first create a new sequence that contains only the letters that are not A, while preserving their original relative order. Once that is done, we just count the letters from left to right until we reach the eleventh position. Breaking the original series into manageable chunks makes this easier.

Step-by-Step Solution:
Step 1: Write the original sequence and mark all As for removal: C, U, B, A, E, D, E, D, A, B, E, B, A, U, C, D, B, C, A, D, B, D, U, B, C, A, C, B, E, D, A.Step 2: Remove every A to form the filtered sequence, preserving the order of the other letters.Step 3: The filtered sequence becomes: C, U, B, E, D, E, D, B, E, B, U, C, D, B, C, D, B, D, U, B, C, C, B, E, D.Step 4: Now count the letters from the left in this new sequence. Position wise: 1 C, 2 U, 3 B, 4 E, 5 D, 6 E, 7 D, 8 B, 9 E, 10 B, 11 U.Step 5: The eleventh letter from the left in the filtered sequence is U.

Verification / Alternative check:
As a check, you can recount the filtered sequence carefully to ensure there are 25 letters and then confirm that the eleventh indeed corresponds to U. Another safe approach is to write the new sequence in a neat row and mark each counted position until you reach the required rank, avoiding any skipping or double counting.

Why Other Options Are Wrong:
• E: Letters E appear at positions 4, 6 and 9, but not at the eleventh position.
• C: C appears at positions 1, 13, 21 and 22 in the filtered series, but none coincide with position 11.
• D: D appears at positions 5, 7, 14, 16 and 25 in the new sequence, not at 11.
• B: B appears at several positions (3, 8, 10, 15, 17, 20 and 23), but again not at the eleventh place.

Common Pitfalls:
A common mistake is to miscount while removing As, either skipping a letter or accidentally leaving an A in the new sequence. Another error is counting positions using the original unfiltered sequence instead of the new one. The correct method is to first perform the removal completely, then work only with the cleaned sequence for any positional counting.

Final Answer:
After removing all As from the given sequence, the eleventh letter from the left in the resulting sequence is U.