Those are all QBasic functions...by (Login qb432l)
Basically, what you have to do is break down the input string to determine what numerical values and operators are in the string. This is intermediate level coding, and if you're not comfortable with it, why not make your calculator a bit more like a conventional one?
Have the user enter numbers and operators (/*+-) in sequence, pressing Enter after each separate input. That way you can calculate the answer as you go along, rather than parse an entire string. For example, 2 (Enter) / (Enter) 5 (Enter) * (Enter) 40 (Enter) = (Enter).