谁能帮我看看这2个数学题 What's the maximum size of a subset,P,of {1,2,3

谁能帮我看看这2个数学题
What's the maximum size of a subset,P,of {1,2,3...50} with the property that no pair of distinct elements of P has a sum divisible by
How many three-digit numbers from 100-999 inclusive have one digit which is the average of the other two?
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