The average score of three boys in Mathematics is 30. When the score of a fourth boy is added to the total score of the three boys, the average is lowered to 25, What is the score of the fourth boy ?
Sum of scores of the 3 boys=avg of scores of the 3 boys×no of boys=30×3=90
Sum of scores including the 4th boy=25×4=100
Thus, the score of the 4th boy=100-90=10
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