It can be observed that 9 students have their blood group as A, 6 as B, 3 as Ad, and 12 as O.
Therefore, the blood group of 30 students of the class can be represented as follows.
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It can be observed clearly that the most common blood group and the rarest blood group among these students is O and AB respectively as 12 (maximum number of
students) have their blood group as O, and 3 (minimum number of students) have their blood group as AB.