Question

a1=5,

a2=15,

a3=125,

a4=1235,

a5=?

Answer 1

$a_{n+1}=12\ldots n \,5$, $n \geq 1,$ $a_1=5$ for $n=0$.

So $a_5=a_{4+1}=12345$

Answer: $12345$.