a(n) = 12*5^n.

Terms

a(0) =12a(1) =60a(2) =300a(3) =1500a(4) =7500a(5) =37500a(6) =187500a(7) =937500a(8) =4687500a(9) =23437500a(10) =117187500a(11) =585937500a(12) =2929687500a(13) =14648437500a(14) =73242187500a(15) =366210937500