[tex]a) \\ n = numarul\;de\;termeni. \\ n= \frac{4021-1}{5} +1=\frac{4020}{5} +1=804+1=\boxed{805\;termeni} \\ \\ b) \\ n=208 \\ t = termenul \;de \;pe \;locul \;208. \\ n=\frac{k-1}{5} +1 \\ 208=\frac{k-1}{5} +1 \\ 208-1=\frac{k-1}{5} \\ k-1=207 \cdot 5 \\ k-1=1035 \\ k = 1035+1 =\boxed{1036}[/tex]
[tex]c) \\ S = \frac{805 \cdot (4021+1)}{2} =\frac{805 \cdot 4022}{2} =805 \cdot 2011=\boxed{1618855}[/tex]