Find the sum of the arithmetic series below.

1. $5+10+15+20+\dots+400$ 

   Hint (a):

   Find the sequence that describes the series.

   Hint 2(a):

   $t(n)=5n$

   Hint 3(a):

   Find which term is $400$.
   
   $(400)=5n$
   $80=n$

   More Help (a):

   Find the sum using the formula from the Math Notes box.

   $S(80)=\frac{80(5+400)}{2}$

   Answer (a):

   $16200$

2. $3+8+13+18+\dots+398$ 

   Hint (b):

   See part (a).

   Answer (b):

   $16040$

3. $80+74+68+62+\dots+14$ 

   Hint (c):

   See part (a).