Alright, let's work through 8405 times 9991 step by step
8405 × 9991 = 9991 × (8405)
+ 9991 × 5 which equals 49955
+ 9991 × 00 that equals 0
+ 9991 × 400 yielding 3996400
+ 9991 × 8000 yielding 79928000

= 83974355
