\[\boxed{\text{6.3}\text{3}\text{9.}}\]

\[2,43 + 2,17 = 4,6\]
\[8,74\ :4,6 = 87,4\ :46 = 1,9\]



\[3,15 + 2,3 = 5,45\]
\[4,36\ :5,45 = 436\ :545 = 0,8\]

\[0,8 + 0,72 = 1,52\]
\(1,52 - 0,78 = 0,74\)

\[3,91\ :2,3 = 39,1\ :23 = 1,7\]

\[9,18 - 4,03 = 5,15\]

\[0,028 + 1,512 = 1,54\]