H_{0}: μ =3100, H_{a}:
μ≠3100. We should use a two-tailed test because μ≠3100
includes values both larger and smaller than 3100.

At a 5% level of significance, decision rule
is to reject H_{o} if the z score is outside the range -1.96 to 1.96.

Under H_{0}, μ =3100 and using
samples standard deviation as an estimate of σ. Since Z=(X_{bar}-μ_{0})/(σ/√*n*)=(3200-3100)/(250/[25])
= 100/50=2.00, which lies outside the range -1.96 to 1.96, we reject H_{0}
at 5% level of significance. While it is close, the evidence suggests that the
mean time until major repair is greater than 3,100 hours