[Guest]

What is the next number of the series

1,9, 18, 25, 27, 21, ?

Edited November 29, 2010 by yorsapat

[araver]

First, see my post about posting homework.

If this is the case, let me give you a hint: most of these problems can be found analyzing how fast your sequence grows i.e. by analyzing differences in consecutive numbers.

E.g. if you see 1,2,4,7 with differences 2-1=1, 4-2=2, 7-4=3 you get how the sequence grows as 1,2,3 is easy to predict.

Another example: if you see 1,2,6,15 with differences 2-1=1, 6-2=4, 15-6=9 you get that the sequence grows with square numbers (1,4,9). Again, it is easier to predict the difference sequence, rather than the original one.

So, have you tried analyzing the differences of this sequence?

Edited November 29, 2010 by araver