Zscore -0.5 standard deviations from mean is -0.5 (1.8) + 12.5 = 11.25 ft
Look at your NEGATIVE z-score table to find .20 (1 - 80% = .20) and find approximately -.84 standard deviations below the mean = .2005 (which means 1-.2005 are ABOVE this).....
55 gm - .84 x 4.2 = 51.47 gm 80% are greater than this
Third one: Look at your POSITIVE z-score table 25 ounces is 1oz/1.3oz/S.D = 0.7692 ~~.77 Standard deviations ABOVE the mean. This corresponds to
.7794 z-score SO 77.94 percent are LESS than this 77.94% x 1200 boxes = 935.28 ~~ 935 boxes are 25 oz or less
Your question is statistics. It has absolutely nothing to do with calculus.
I apologize. I thought this site may be able to help. I did not intend to offend.
You didn't offend. I am just stating that your title is incorrect.
It is better if titles reflect the content of the question.
And the content of this questions is
Statistics OR Normal distributions OR Using Z scores.
Personally I think 'Using Z scores with normal distributions' would be a very good title.