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3. A list of weights follows the normal curve with an average of 160 lb and a standard deviation (SD) of 20 lb. For each question, draw the normal curve and indicate the area of interest.

a) About what percentage of the people weigh less than 135 lb?

b) What percentage of weights is between 150 and 190 lb?

c) Find the 30th percentile weight. Explain.

Answer :

a) Percentage of people weighing less than 135 lb is 10.56% , b) percentage of weights between 150 and 190 lb is 0.6247 or about 62.47%, c) 30th percentile weight is is 149.6 lb.

a) To find the percentage of people weighing less than 135 lb, we first calculate the Z-score.

Z-score = (X - Mean) / SD = (135-160) / 20 = -1.25. Using a standard Z-score table, the area corresponding to this Z-score is approximately 0.1056, or 10.56% of people weigh less than 135 lb.

b) To find the percentage of weights between 150 and 190 lb, we similarly find the Z-scores for both weights. For 150 lb,

Z-score = (150-160) / 20 = -0.5. For 190 lb, Z-score = (190-160) / 20 = 1.5.

Using a Z-score table, the areas corresponding to these scores are approximately 0.3085 for -0.5 and 0.9332 for 1.5.

So,the percentage of weights between 150 and 190 lb is 0.9332 - 0.3085 = 0.6247, or about 62.47%.

c)The 30th percentile weight corresponds to a Z-score of approximately -0.52. Using the formula Weight = Z(SD) + Mean, we find the 30th percentile weight to be approximately -0.52(20) + 160 = 149.6 lb.

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