If you get stuck trying to solve then looking here may give you some helpful hints and tips.
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In this case A must contain 1 as all other numbers have been placed. | 
In this case A cannot be 6 or 5 since they are already around circle X. A cannot be 2 or 3 since they are already around circle Y and A cannot be 4 since it is already around circle Z.Therefore it must be 1 |
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In this case A must be 6 because it is the only place around circle Y which would not cause 6 to be placed twice around circles X and Z.
Then B must be 1 other else it would have to be placed twice around circle X.
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In this case A must be 1. |
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In this case A must be 6. This is because it is the only place that 6 can be placed around circle X without causing 6 to be placed twice around other circles.
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