Cross-multiplication

General formula (from World Mental Calculation)

Start with the rightmost digit of each number, then
- calculate their product,
- write down the units digit, and
- carry the tens digit for the next stage.

For every subsequent digit of the answer:
- take the carried number from before, 
- add all products of the same magnitude by working systematically, 
- write down the units digit in the final answer, and
- carry the rest for the next stage.

Example solve coming soon!