What button on a helix oxford calculator...?

I need to know what button on my new helix oxford calculator converts decimals into fractions? I have my maths GCSE tomorrow so seriously need to know now. Plus, what button can you press for an index number of any number? I know you can press the button below the 'down arrow' button to square a number and you can press the button below the 'on' button to cube it but what if they ask for example what is 9 to the power of 21 or something? We have went over questions like that in school so it is possible for the examiners to put that in the paper so what button enables you to do that? please give me answers asap no joke x