Dynamic Evaluation Of Expression Operator (was Substitution)

Hi I am trying to do something like the following:

DECLARE @Operator varchar(1)
DECLARE @Rate float
DECLARE @Quantity float
DECLARE @Converted float

SET @Quantity = 6
SET @Operator = '/'
SET @Rate = 2
SET @Converted = 0

@Converted = (@Quantity substituteTheValueOfThis(@Operator) @Rate)

PRINT @Converted

so that the output would be 3

The reason I need to do it like this is that @Operator will change at runtime...

Any suggestions appreciated, I have looked at EXEC sp_execsql but somehow can't get the syntax right.