7-2. Consider the compressive strength data in Table 62. What proportion of the

Question

7-2. Consider the compressive strength data in Table 62. What proportion of the specimens exhibit compressive strength of at least 1379 KN/m? TABLE. 6-2 Compressive Strength (in psi) of 80 Aluminum-Lithium Alloy Specimens 143 105 97 245 163 221 154 228 186 174 199 115 193 167 180 167 176 158 156 121 120 183 153 174 154 190 76 101 142 149 181 168 158 208 133 135 172 133 160 194 184 165 145 160 150 123 180 178 157 134 218 199 160 196 169 158 148 163 87 176 150 135 175 201 237 170 7-12. The amount of time that a customer spends waiting at an airport check-in counter is a random variable with mean 8.2 minutes and standard deviation 6 minutes. Suppose that a random sample of n=36 customers is observed. Find the probability that the average time waiting in line for these customers is (a) Less than 10 minutes (b) Between 5 and 10 minutes (c) Less than 6 minutes

Explanation / Answer

7.12) here std error of mean =std deviation/(n)1/2 =6/(36)1/2 =1

a) probability less then 10 minutes =P(X<10)=P(Z<(10-8.2)/1)=P(Z<1.8)=0.9641

b)P(5<X<10)=P((5-8.2)/1<Z<(10-8.2)/1)=P(-3.2<Z<1)=0.9641-0.0007 =0.9634

c) P(X<6)=P(Z<(6-8.2)/1)=P(Z<-2.2)=0.0139

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