Question 314405
Hey,

First of all, this is an arithmetic sequence. The tn thing means the specific number in the sequence. Each specific number in an arithmetic sequence is:

t(n)=a(first term)+(n-1)d(the difference)

The (a) in this case is 4
The (d) in this case is also 4, the common difference between the terms

so, if t(n)=256, then:

a) 256=4+(n-1)4   =>  252=(n-1)4   => 63=n-1   => n=64

b) The sum for an arithmetic sequence is S(n)= (n/2)(a1+a(n))
therefore:

S(64)=(64/2)(4+256)=8320

I hope this helps!