An investment has an expected return of $11$ percent per year with a standard deviation of $24$

percent. Assuming that the returns on this investment are at least roughly normally

distributed, how frequently do you expect to earn between $13$ percent and $35$ percent? How

often do you expect to earn less than $13$ percent?

I have the answer to this question stated below I just don't understand how it being plus or minus one standard deviation why it would be $2/3$rds of the time and would like it explained, I also would like to know what the answer it would be if it were plus or minus two standard deviations.

The range of $13$ percent to $35$ percent is plus or minus one standard deviation, so about two$-$thirds

of the time, or about two years out of three.

In one year out of three, you will be outside this range, implying that you will be below it $($less than

$13$ percent$)$ about one year out of six and above it $($greater than $35$ percent$)$ about one year out of six