No, `89.3 + / 2`

is not syntactically correct, so
cannot be an arithmetic expression.

Notice that `89.3 + / 2`

is made of
correct components—numbers and operators—but that
they are put together incorrectly.

When two or more arithmetic operators are used in the same arithmetic expression it is sometimes not clear what to do. For example:

1 + 2 * 3 = ?

This could mean either of two things, depending on what operation is done first:

1 + 2 * 3 = 3 * 3 = 9 -----

OR

1 + 2 * 3 = 1 + 6 = 7 -----

It is not clear if + or if * should be done first. Which one is done first makes a difference in the result.

What is the usual (paper-and-pencil) meaning of

1 + 2 * 3