If you want to solve this problem using formulas, there are two important formulas:
t1 = first term = -5
tn = nth term = last term = -5
n = numbr of terms
Sn = sum of the n terms
tn = t1 + (n - 1)d ---> 65 = -5 + (n - 1)(5)
65 = -5 + 5n - 5
65 = -10 + 5n
75 = 5n
n = 15
Sn = n(t1 + tn)/2 ---> Sn = 15(-5 + 65)/2
Sn = 450
So ur answer rounds up to 450
Letter c
:)
hope i helped
~Luis