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.

Which Operator is done First?

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   
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1   +   2 * 3    =   1 + 6  =  7
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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