Which number should come next in the series : 1, 3, 6, 10, 15?
A
18
B
20
C
21
D
25
Correct Answer: 21
This is the Triangular Number Sequence: 1, 3, 6, 10, 15 ... We can use xn to mean "dots in triangle n", so we get the rule: Rule: xn = n(n + 1)/2 the 6th Triangular Number is x6 = 6(6 + 1)/2 = 21 so,ans is 21