KAPLAN 2009 Premier Program Math

[fezario]

hi guys, I am starting to look at the Kaplan Premier Program book and i am a little confused about something.
In many of the questions I have encountered the actual calculations required are quite hard to do without a calculator.
In fact in some sections of the answers the book suggests using a calculator, which is not allowed in the exam.
Am I seriously supposed to work out sums like 195/0.65 = 300 in my mind during the exam or are the calculations easier on the actual GMAT?
Please help so I can prepare accordingly.