Question 1185010
z=(x-mean)/sd
a.z < (10-12.4)/2.1
 =-1.14
that probability is 0.1271
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b. z > (20-12.4)/2.1
=7.6/2.1
=3.62
that probability is 0.00015
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90th percentile is z=+1.282
1.282=(x-12.4)/2.1
2.69=x-12.4
x=15.1 minutes