[tex] \frac{a + b + c + d}{4} = 8.5 = > a + b + c + d = 8.5 \times 4 = 34 \\ \frac{8.5 \times 3 + 1 \times 8}{3 + 1} = \frac{25.5 + 8}{4} = \frac{33.5}{4} = 8.375 \\( 8.5 \times 3 + 10 ) \div 4= 35.5 \div 4 = 8.875 \: sau \:( 8.5 \times 3 + 9) \div 4 = 34.5 \div 4 = 8.625[/tex]