а) \(2\frac{7}{9}+6.9=2\frac{7}{9}+6\frac{9}{10}=9+\frac{7}{9}+\frac{9}{10}=9+\frac{70+81}{90}=10\frac{61}{90}\).
б) \(0.6+\frac{3}{5}=\frac{3}{5}+\frac{3}{5}=\frac{6}{5}=1\frac{1}{5}\).
Ответ: а) \(10\frac{61}{90}\); б) \(1\frac{1}{5}\).