Question

pls tell how the answer came.
Q. What's the next no.
1 ,3 ,7 ,13 ,____
Ans. 21
Prove that Ans is 21.

Answer 1

THE ANS.IS 21 BECAUSE THERE IS A SERIES OF GAP OF 2,4,6 .THEREFORE 8 WILL BE THE NEXT.ND 13+8=21
HENCE ,PROVED....

Answer 2

1 + (1*2) = 3

3 + (2*2) = 7

7 + (3*2) = 13

So, 13 + (4*2) = 21