\[\boxed{\mathbf{579.}\mathbf{\ }}\]
\[1)\ 9,88\ :a = 3,8\]
\[a = 9,88\ :3,8\]
\[a = 988\ :380\]
\[a = 2,6.\]
\[3,8 - b = 1,74\]
\[b = 3,8 - 1,74\]
\[b = 2,06.\]
\[1,74 \cdot c = 6,09\]
\[c = 6,09\ :1,74\]
\[c = 609\ :174\]
\[c = 3,5.\]
\[2)\ 6,2 \cdot x = 17,36\]
\[x = 17,36\ :6,2\]
\[x = 1736\ :620\]
\[x = 2,8.\]
\[17,36 + y = 20,1\]
\[y = 20,1 - 17,36\]
\[y = 2,74.\]
\[20,1\ :z = 1,5\]
\[z = 20,1\ :1,5\]
\[z = 201\ :15\]
\[z = 13,4.\]