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Hi
Normal Distribution: µ = 7.4 and σ = 2.8
Using TI or similarly an inexpensive calculator like an Casio fx-115 ES plus
a. (100 - 91)/2 = .045% Invnorm(.045) = -1.6954
7.4 ± (1.6954*2.8) = 7.4 ± 4.7 = 2.7 to 12.1
b. P x > 16) = normcdf(16, 9999, 2.8, 7.4) = .00107 Not Likely
Wish You the Best in your Studies.