No, you can't take a calculator for the exam. If there is only one answer to be selected( look for radio-button ), then you can use the method of elimination to arrive at the best answer. If you have to select more than one, you will have to do it the hardway - using paper and pencil. These kind of questions might take more than a minute to figure out, so
you should mark them( there will be a little checkbox on the top-left corner of the screen ) and review them later.
Having said that, I'm sure you can formulate some easy methods to convert from one number system to another. Also make sure you know the ranges and can make an educated guess whether the number that is being assigned fits in to a specific range.
Hope that helps,
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Ajith Kallambella M. Sun Certified Programmer for the Java�2 Platform.
IBM Certified Developer - XML and Related Technologies, V1.