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Sagot :
For each probability, transform the normal random variable X with mean 19 and s.d. 4.04 to a standard normal random variable Z with mean 0 and s.d. 1, using the rule
Z = (X - 19) / 4.04
(a) Pr[15 ≤ X ≤ 20] = Pr[(15 - 19)/4.04 ≤ (X - 19)/4.04 ≤ (20 - 19)/4.04]
… ≈ Pr[-0.9901 ≤ Z ≤ 0.2475]
… ≈ Pr[Z ≤ 0.2475] - Pr[Z ≤ -0.9901]
(since Z is a continuous random variable)
… ≈ 0.4367
(b) Pr[X > 20] = Pr[(X - 19)/4.04 > (20 - 19)/4.04]
… ≈ Pr[Z > 0.2475]
… ≈ 1 - P[Z ≤ 0.2475]
(taking the complement probability)
… ≈ 0.4023
(c) Pr[X < 15] = 1 - Pr[15 ≤ X ≤ 20] - Pr[X > 20]
(also taking the complement)
… ≈ 0.1611
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