Arithmetic Sequences Help!

I'm really stuck on how to solve an Arithmetic Sequence problems when it only gives me one term and the difference as well as how many terms the sum needs to go to. Example: An A.P has 3rd term of 45 and difference of -1.1. Find the sum of the first 20 terms. How do I do this?

The nth term = a+(n-1)*d
45 = a + (3-1)*-1.1
a = 47.2
20th term = 47.2+19*-1.1 = 26.3
Sum = (47.2 + 26.3)*20/2 = 735

Sum of the first n terms of an AP with first term a and difference d is

S = n( 2a + (n-1)d) / 2

The general term of an AP is => a + (n-1)d so third term is a + (3-1)(-1) = 45
(Its not clear if d is -1 or -1.1 . I used -1. if its not use -1.1)
a - 2 = 45
a=47.

So we have a =47, d= -1 and n=20. and S = S = n( 2a + (n-1)d) / 2

once we plug in the values we have the answer

S = 20(2*47+(20-1)*(-1))/2 = 750.

Given:
a(3) = 45
d = -1.1

Use the formula for an arithmetic sequence:
a(n) = a(1) + d(n - 1)
a(n) = a(1) − 1.1(n - 1)

Find a(1) using the given a(3):
a(3) = a(1) − 1.1(3 - 1) = 45
a(1) − 1.1(2) = 45
a(1) − 2.2 = 45
a(1) = 45 + 2.2
a(1) = 47.2

Find the full expression for the sequence using the formula (again) and the value of a(1):
a(n) = 47.2 − 1.1(n - 1)
Which can be simplified to:
a(n) = 47.2 − 1.1n + 1.1
a(n) = 48.3 − 1.1n

Sum formula:
S(n) = n/2 * (a(1) + a(n))
S(20) = 20/2 * (47.2 + a(20))

... [Sub-step] Find a(20):
... a(20) = 48.3 − 1.1(20)
... a(20) = 48.3 − 22
... a(20) = 26.3

S(20) = 10 * (47.2 + 26.3)
S(20) = 10 * (73.5)
S(20) = 735

Ans :Common difference(d) = -1.1
Third term (A3) = 45
So : A+2d = 45
Therefore the First term : A = 45 - 2(1.1) = 42.8
No of terms = 20
Apply : Sum of n terms : Sn = n/2 (2a+(n-1)d)
=> S20 = 20/2*(42.8+(19)[1.1])
=> S20 = 10*(42.8+(19)[1.1])
=> S20 = 10*(63.7)
=>S20 = (637)
Therefore : Sum of the first 20 terms = 637

I wish you will appreciate my answer.

t3 = 45, so t2 = 46.1 and t1 = 47.2
tn = 47.2 - (n-1)*1.1

t20 = 47.2 - 19*1.1 =26.3

t20 + t1 = 73.5, so average is 36.75

Multiply by 10 = 367.5