Missing digit for divisibility by 3 or 9 (digit sum)

This type of question, asked 4 times: 21 Jul 2023 S4, 17 Sep 2024 S2, 24 Sep 2024 S1, 24 Sep 2024 S3. All 4 questions below, oldest first.

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Question 1
21 Jul 2023 S4
54 $Find the value of a to make 6234a6 divisible by 9.$
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Question 2
17 Sep 2024 S2
51 $Find the least value of x for which 57x716 is divisible by 9.$
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Question 3
24 Sep 2024 S1
51 $Find the greatest value of ‘k’ in a 6-digit number 6745k2 such that the number is divisible by 3.$
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Question 4
24 Sep 2024 S3
74 $If 436P5 is divisible by 3, then the largest value of P is:$
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Question 1