The formula to calculate a z$-$score is z $=($x $-)/\backslash$sigma

The formula to calculate x from a z$-$score is x $=+$ z$\backslash$sigma

On a Quiz, scores were normally distributed, with a mean of $81$ and a standard deviation of $4.$

What is the probability that a randomly selected person has a quiz score less than $80?$

$(4$ decimal places only, no leading $0,$ NOT as a percent$)$