No problem, we've got 9920 and 8640 to multiply
9920 × 8640 = 8640 × (9920)
+ 8640 × 0 what gives us 0
+ 8640 × 20 yielding 172800
+ 8640 × 900 giving us 7776000
+ 8640 × 9000 yielding 77760000

= 85708800
