Using frontend multiplication simplifies the task.

1
Using frontend multiplication simplifies the task.
2 4 6
X   5 8

2
2 4 6 2 0 0
X   5 8 X   5 0
2 4 6 2 0 0
X   5 8 X   5 0
1 0 0 0 0
2 4 6 2 0 0 4 0 6
X   5 8 X   5 0 X   5 0 X   5 0
1 0 0 0 0
2 4 6 2 0 0 4 0 6
X   5 8 X   5 0 X   5 0 X   5 0
1 0 0 0 0 2 0 0 0 3 0 0
2 4 6
X   5 8
1 0 0 0 0
2 0 0 0
3 0 0
2X5 10
2
X   5
1 0
2 0 0
X   5 0
1 0 0 0 0
Tack on the trailing zeros
3
2 4 6 2 0 0 4 0 6
X   5 8 X   5 0 X   5 0 X   5 0
1 0 0 0 0 2 0 0 0 3 0 0

2 0 0 4 0 6
X     8 X     8 X     8
1 6 0 0 3 2 0 4 8
1 0 0 0 0
2 0 0 0
3 0 0
1 6 0 0
3 2 0
      4 8
1 4 2 6 8
Add up the factors.