14.5
Step 1:
Find the constant added which leads to the following term.
. Since 2 terms are given the 6th and the 10th term and they have a difference of 4 terms between:
a(6) + 4d = a(10)
with d representing the constant in question and n the terms position.
Step 2:
Find the required terms by starting from the known ones and adding or subtracting depending on their position.
. Since d = 1.25 and the 4rth term is 2 terms below the known 6th:
a(4) = a(6) - 2d
and keep the sequence until each of the first 4 terms are found.
Step 3:
Sum the first 4 terms found:
. a(1) + a(2) + a(3) + a(4) = Sum