Index of 10^n within the sequence of the numbers of the form 5^i*10^j.

Terms

a(0) =1a(1) =3a(2) =6a(3) =11a(4) =17a(5) =25a(6) =34a(7) =45a(8) =57a(9) =70a(10) =85a(11) =101a(12) =119a(13) =138a(14) =159a(15) =181a(16) =204a(17) =229a(18) =255a(19) =283a(20) =312a(21) =343a(22) =375a(23) =408a(24) =443a(25) =479a(26) =517a(27) =556a(28) =597a(29) =639