Alright, let's work through 9890 times 640 step by step
9890 × 640 = 640 × (9890)
+ 640 × 0 that is equal 0
+ 640 × 90 that results in 57600
+ 640 × 800 giving us 512000
+ 640 × 9000 that results in 5760000

= 6329600
