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P (red or pink) = P(red) + P(pink) - P(red and pink)
P(red) = 7/28
P(pink) = 9/28
P(red and pink) = 0 (red and pink marbles are mutually exclusive)
P(red or pink) = 7/28 + 9/28 = 16/28 = 4/7
Therefore, P(red or pink) = 4/7