Answer:
12.5
Explanation:
Let the first term T1 be a,
second term T2 = a+d where d is the common difference
If the third term and each term thereafter is the average of the two terms immediately preceding it then;
T3 = T2+T1/2
T3 = a+d+a/2
T3 = 2a+d/2
Since the first term of the sequence is 20 and the second term is 8;
a = 20 ... 1
a+d = 8 ... 2
from 2; d = 8-20
d = -12
T3 = 2(20)+(-12)/2
T3 = 40-12/2
T3 = 28/2
T3 = 14 (This value is an integer)
We will proceed to check the fourth term of the sequence.
T4 = T3+T2/2
T4 = 14+8/2
T4 = 22/2
T4 = 11 (This is also an integer)
We will proceed to check the value of fifth term of the sequence.
T5 = T4+T3/2
T4 = 11+14/2
T4 = 25/2
T4 = 12.5 (This is not an integer)
This shows that the value of the first term in the sequence that is not an integer is 12.5