The store buys a helmet for $19.00 and sells it for $31.50. What percentage was the markup?
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Arccos is the same thing as inverse cosine which is used when you are looking for an angle measure. Make sure your calculator is in degree mode. Hit 2nd button then cos and you will see this on your screen: 
and it is following the parenthesis there that you will enter your decimal. The inverse cosine of .67 is an angle measure of 47.93°