$6\mathrm{.}13$ A group of individuals with standard health con$-$ ditions are put on an experimental diet for one month. The gain in weights of these individuals after a month is normally distributed. They average $1450$ grams, with a standard deviation of $250$ grams. Find the probability that the gain in weight for a given individual will be

$($a$)$ more than $1700$ grams; $($b$)$ less than $1250$ grams;

$($c$)$ between $1100$ and $1600$ grams.