[tex]M_a(a; b) = \frac{a+b}{2}= \frac{12}{10} \\ \\ => (a+b) = 2*\frac{12}{10} = \frac{12}{5} \\ \\ M_a(a; b;c) = \frac{a+b+c}{3}= \frac{27}{15} \\ \\ => (a+b+c) = 3*\frac{27}{15} = \frac{27}{5} \\ \\c=(a+b+c)-(a+b)=\frac{27}{5}-\frac{12}{5} = \frac{27-12}{5}=\frac{15}{5}=3[/tex]