Hi. Just another of many alternatives:
JoinNumbers[v_List] := Fold[#1*10^IntegerLength[#2] + #2 &, 0, v]
JoinNumbers[{1,23,456,7890,12345}]
123456789012345
Example = {{97,98,99},{65,66,67},{90,89}};
JoinNumbers/@Example
{979899,656667,9089}
I've tried every possible combinations of Join, Flatten, Level, IntegerDigits, FromDigits etc
Another possible solution:
fx[v_List] := FromDigits /@ Flatten /@ Map[IntegerDigits, v]
fx[Example]
{979899,656667,9089}
Note: Fold might be a little faster:
Do[fx[Example],{100000}]//Timing
{2.59130999999999,Null}
Do[JoinNumbers/@Example,{100000}]//Timing
{1.86826099999999,Null}
= = = = =
HTH :>)