LSBB Practice Test Question - Confidence Intervals

I received this test question,and must have solved it incorrectly - or used the wrong formula somewhere. The explanation did not help, so I looking for some other advice on where I went wrong:


I used Z table (since samples > 30).

xbar +/- Z (of alpha/2) * [stddev / sqrt(n) ]

20 +/- 1.96 * [4 / sqrt(64) ]


UCL = 20.98; LCL = 19.02


Not sure where I went wrong....

THANKS for your help


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LSBB Practice Test Question - Confidence Intervals