a puxxle

[Jud McCranie]

At 11:00 PM 4/13/2006, N. J. A. Sloane wrote:
>Dear Seqfans:
>Someone just sent this in.  I gave it an A-number.  But I can't
>add it to the OEIS until the missing term is filled in. Any ideas?
>The differences are suggestive
>
>%I A117713
>%S A117713 1,3,8,18,__,89,189
>%N A117713 This sequence was a question on a civil service test a friend 
>of mine took. They had to fill in the blank number. Is there an answer to 
>this??.
>%O A117713 0,2
>%K A117713 nonn
>%A A117713 Louis Ciotti (lciotti(AT)twcny.rr.com), Apr 13 2006
>
>Differences: 2 5 10 a b 100 , but a+b = 71, which is ugly.

Yes, it seems more likely that the differences would be 2, 5,10, 20, 50, 
100, but that doesn't fit.