High School

The temperature recorded by a certain thermometer, when placed in boiling water (the true temperature is 100 °C), is normally distributed with a mean of [tex]\mu = 99.8 \, \text{°C}[/tex] and a standard deviation of [tex]\sigma = 0.1 \, \text{°C}[/tex]. Answer the following questions:

1. What is the probability that the thermometer reading is less than 99.5 °C?
2. What is the probability that the thermometer reading is greater than 100 °C?
3. What is the probability that the thermometer reading is within ± 0.05 °C of the true temperature of 100 °C?
4. What is the third quartile of the recorded temperature values?

Answer :

Probability that Thermometer Reading is less than 99.5°C:

We are given that temperature recorded by a certain Thermometer when placed in boiling water (the true temperature is 100 °C) is normally distributed with mean µ = 99.8°C and Standard Deviation σ = 0.1°C.

We need to find the probability that thermometer reading is less than 99.5 °C.

Using the z-value formula: z = (x-µ)/σFor x = 99.5°C, µ = 99.8°C, and σ = 0.1°C,z = (99.5 - 99.8) / 0.1= -3So, P(x < 99.5) = P(z < -3) = 0.0013 [using normal distribution table]

Probability that thermometer reading is greater than 100°C:

Using the z-value formula: z = (x-µ)/σFor x = 100°C, µ = 99.8°C, and σ = 0.1°C,z = (100 - 99.8) / 0.1= 2

So, P(x > 100) = P(z > 2) = 0.0228 [using normal distribution table]

Probability that thermometer reading is within ± 0.05 °C of the true temperature of 100 °C:

Using the z-value formula: z1 = (x1-µ)/σ, z2 = (x2-µ)/σFor x1 = 99.95°C, x2 = 100.05°C, µ = 100°C, and σ = 0.1°C,z1 = (99.95 - 100) / 0.1= -0.5, z2 = (100.05 - 100) / 0.1= 0.5P(99.95 < x < 100.05) = P(-0.5 < z < 0.5) = 0.3829 [using normal distribution table]

Third quartile of the recorded temperature values:

Using the z-value formula: z = (x-µ)/σ

For third quartile, 75th percentile is used

. As we know that 75% of the area is below

So the remaining 25% will be above it.

Using the Normal Distribution Table, the z-value corresponding to 0.25 probability is 0.67.

For µ = 99.8°C, σ = 0.1°C and z = 0.67,

we get, x = µ + zσ= 99.8 + 0.67(0.1)= 99.8665 °C

Therefore, the third quartile of the recorded temperature values is 99.8665°C.

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