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The quantity you want is the area under the standard normal curve (with mean 100 and std dev 15) between 110 and 120.
According to my TI-83 calculator,
normalcdf(110,120, 100, 15) = 0.16, or 16%.
The total cost for this item is $18.19.
To find the sales tax of something, you move the decimal two places to the left on the percent. Then you multiply the items cost and the tax together, which gets you $1.19. Add the original amount with 1.19 and you get 18.19.
