Multiplications ( Teknik Pisah-Gabung)

Well 6-digit numbers may be a stretch. Typically the only people who can accurately multiply two 6-digit numbers (with not many 0's as digits) in their heads in a reasonable amount of time are considered savants.

But maybe it would help to explain how to do 3-digit numbers.

Take, for example 123 * 456.
= (100 + 20 + 3)(400 + 50 + 6)
= (100*400 + 20*400 + 3*400) + (100*50 + 20*50 + 3*50) + (100*6 + 20*6 + 3*6)
= (40000 + 8000 + 1200) + (5000 + 1000 + 150) + (600 + 120 + 18)
= (49200 + (6150) + (738)
= (55350) + (738)
= 56088

All of these multiplications you can do in your head, and just keep a running sum.