Answer:
k = 11 or 20
Explanation:
The sum of terms of an arithmetic sequence is given by ...
Sn = (a1 +an)(n/2)
We want to find the value of n for a particular series to have a particular sum.
__
parameters
The series of interest is defined by ...
an = 48 -3n
so, the first term is ...
a1 = 48 -3(1) = 45
The k-th term is ...
ak = 48 -3k
__
sum
Then the sum is ...
Sk = (45 +(48 -3k))(k/2)
We want this value to be 330, so we have ...
330 = (93 -3k)(k/2)
__
solution
220 = (31 -k)k . . . . . . . . multiply by 2/3
k² -31k +220 = 0 . . . . . in standard form
We want factors of 220 that total 31.
220 = (1)(220) = (2)(110) = (4)(55) = (5)(44) = (10)(22) = (11)(20)
The last of these pairs totals 31, so we have ...
(k -11)(k -20) = 0 . . . . . factored form of the equation
k = 11 or k = 20 . . . . . values of k that make the factors zero
Possible values of k are 11 and 20.