1.

The time until first failure of a brand of inkjet printers is normally distributed with a mean of 1,500 hours and a standard deviation of 200 hours. What proportion of printers fails before 1000 hours? (a) 0.0062 (b) 0.0668 (c) 0.8413 (d) 0.0228

Answer»

(a) 0.0062 

µ = 1500, σ = 200 

P(X < 1000) 

= P(Z < (1000 - 15000)/200) 

= P(Z < -2.5) 

= P (Z > 2.5) = 0.5 – P (0 < Z < 2.5) 

= 0.5 – 0.4938 

= 0.0062



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