My grandaughter has just sent me the following. I am clueless. Anyone able to tell me how to calculate. 
Many thanks. dd.
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I am 100 percent certain that you could find this equation, and therefore the answer, online… 100 percent!
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I smell a Derleydoo Rat.
No rats, MagsJ, whatever that means. What you have here is a genuine request for assistance. This site has self-proclaimed genius in abundance.
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Ah… ok.