(a)  In part (a), they want you determine the probability that a randomly selected quartz 
     time piece will have a replacement time less than 12.1 years.


     It is the same as to determine the area under the given normal curve 
     on the left of the time mark of 12.1 year.


     Use the standard function normalcdf of your regular calculator TI-83/84

                         z1    z2    mean   SD    <<<---===  formatting pattern
         P = normalcdf(-9999, 12.1,  15.9,  1.4)


     You will get the value P = 0.0033  (rouned).    ANSWER


     Alternatively, you may use free of charge online calculator at this web-site

         https://davidmlane.com/hyperstat/z_table.html

     (use it in the mode "Area from a value").


     It has clearly designed interface, so even a beginner student can learn it and use easily.

     Online calculator will give you the same answer.



(b)  In part (b), they want you determine the time mark on the number line such that
     only 0.9% = 0.009 of the quartz time pieces will be replaced before the warranty expires.


     It is the same as to determine the time mark on the number line such that 
     the area under the normal curve on the left of this mark would be 0.009.


     Use the standard function invNorm of your regular calculator TI-83/84

                          area    mean   SD    <<<---===  formatting pattern
         years = invNorm(0.009,  15.9,  1.4)


     You will get the value years = 13.534,  or 13.5 years,  rounded.    ANSWER


     Alternatively, you may use the same free of charge online calculator at this web-site

         https://davidmlane.com/hyperstat/z_table.html

     (this time use it in the mode "Value from an area").


     It has clearly designed interface, so even a beginner student can learn it and use easily.

     Online calculator will give you the same answer.