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6. 10
There are 8 groups of three numbers round the diamond totalling 15;
10 + 3 + 2 = 15,
8 + 2 + 5 = 15 etc

12. S
They are the 1st letters of the numbers 1-2-3-4-5-6.

Start at 12 and move clockwise. Opposite segments as indicated in the diagram below are plus 1, then plus 2 etc.