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**brumby_3** They're not one-for-one because one if f(4) and the other is f(11) and they're not the same number, right - one is 4 and one is 11.

For ii), you've pretty much wrote it, f(ai) and f(11) are the same, and do equal 15.

For iii) So you choose six numbers, you can see it looking at the set P, there's always going to be one pair that equals 15 because there will always be a match, say if i choose 3, 4, 5, 6, 7, and 12... 3 and 12 match, or if I choose 5, 6, 7, 8, 9, 10, 11.. then 7 + 8 = 15.

Thanks for your help and please tell me if my answers are right or a bit sketchy