Question 898601
3,9,15,__,57
This is an arithmetic sequence defined by the formula for the nth term in the sequence as
Xn = a + d(n - 1) where a is the first term and d is the difference
in our example a = 3 and d = 6 ( 9 - 3)
now for the fourth term
X4 = 3 + 6*(4 - 1)
X4 = 3 + 18
X4 = 21