A major credit card company has determined that its customers charge an average of $$260$ per month on their accounts with a standard deviation of $$20.($You may need to use the appropriate appendix table or technology to answer this question. Round your answers to two decimal places.$)$

$($a$)$ What percentage of the customers charges more than $$255$ per month? $\%$

$($b$)$ What percentage of the customers charges less than $$247$ per month? $\%$

$($c$)$ What percentage of the customers charges between $$244$ and $$305\mathrm{.}60$ per month? $\%$