1. plumbing: aaa plumbing charges $60 per hour plus a $100 travel charge
for a service call. rooter plumbing charges $80 per hour and no travel fee.
how long must the service call be for each to charge the same amount?

Afactory makes propeller drive shafts for ships. a quality assurance engineer at the factory needs to estimate the true mean length of the shafts. she randomly selects four drive shafts made at the factory, measures their lengths, and finds their sample mean to be 1000 mm. the lengths are known to follow a normal distribution whose standard deviation is 2 mm. calculate a 95% confidence interval for the true mean length of the shafts. input your answers for the margin of error, lower bound, and upper bound.