\[\boxed{\text{84.}}\]
\[7\ 000 - 1\ 000 = 6\ 000\]
\[2\ 800 - 300 = 2\ 500\]
\[4\ 000 - 1\ 500 = 2\ 500\]
\[6\ 200 + 600 = 6\ 800\]
\[3\ 100 + 3\ 700 = 6\ 800\]
\[7\ 000 - 1\ 500 = 5\ 500\]
\[4\ 000 + 2\ 000 = 6\ 000\]
\[3\ 000 - 500 = 2\ 500\]
\[\boxed{\text{85.}}\]
\[9\ 000 - 2\ 000 = 7\ 000\]
\[1\ 700 - 200 = 1\ 500\]
\[5\ 800 - 2\ 600 = 3\ 200\]
\[2\ 400 + 3\ 600 = 6\ 000\]
\[10\ 000 - 1\ 000 = 9\ 000\]
\[7\ 300 + 500 = 7\ 800\]
\[3\ 900 - 1\ 700 = 2\ 200\]
\[4\ 600 + 4\ 000 = 8\ 600\]
\[\boxed{\text{86.}}\]
\[5\ 200,\ 6\ 400,\ 4\ 800,\ 4\ 900,\ 3\ 990.\]
\[\boxed{\text{87.}}\]
\[3000,\ 7\ 200,\ 4\ 300,\ 6\ 400,\ 8\ 100.\]