Arithmetic Series

3.5   + 2.7  + 1.9.......

The common difference is  - .8

The 30th term, a₃₀,  is given by  :

a₃₀   =  3.5 - .8(29)   =  -19.7

And the sum  is given by

[a₁ + a₃₀] * n / 2           where  a₁   =  3.5,   a₃₀  =  -19.7    and n   = 30

[ 3.5 +   -19.7] * 30 / 2   =   -243