Astudent measured the lead content of a paint sample 4 times. the standard deviation of the measurements was found to be 1.1% of the average. can this student be 90% confident that the true value is within 1.8% of the measured average?

yes. 1.8% is 1.8/1.1 = 1.64 standard deviations from the mean. So looking at the table of standard deviation and confidences, I see for the 90% value a value of 1.644854Ď which is slightly higher than the computed value of 1.64. So yes, the student can be 90% confident that the true value is within 1.8% if the measured average.

Yes, two vertical lines can be complementary