here P(a bulb last longer than 10
hours)=P(X>10)=e-t=e-(1/10)*10
=0.3679
hence expected number of bulbs last longer than 10 hours=np=600*0.3679=220.74
also std deviation =sqrt(np(1-p))=11.8123
for normal distribution z score =(X-mean)/std deviation
hence from normal approximation and continuity correction:
P(at least 250 bulbs last longer than 10 Hours)=P(X>=250)=1-P(X<=249)
=1-P(Z<(249.5-220.74)/11.81)=1-P(Z<2.43)=1-0.9925=0.0075
10. (10 points) One has 600 light bulbs whose life times are independent exponential random variables...
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