Alright, let's work through 21405893 times 80398728 step by step
21405893 × 80398728 = 80398728 × (21405893)
+ 80398728 × 3 that results in 241196184
+ 80398728 × 90 producing 7235885520
+ 80398728 × 800 yielding 64318982400
+ 80398728 × 5000 giving us 401993640000
+ 80398728 × 00000 resulting in 0
+ 80398728 × 400000 resulting in 32159491200000
+ 80398728 × 1000000 which equals 80398728000000
+ 80398728 × 20000000 that equals 1607974560000000

= 1721006568904104
