Find the difference between the greatest and the smallest numbers that can be written by using the digits $7, 0, 8, 3, 1$ only once?

Round off to the nearest ten, hundred and thousand: $345$

Estimate the following:

$728+6381$

Estimate the following:

$72\times 349$

Estimate the following:

$8465-1364$

Estimate the following:

$1235÷11$