Alright, let's work through 7800 times 5021 step by step
7800 × 5021 = 5021 × (7800)
+ 5021 × 0 that is equal 0
+ 5021 × 00 that results in 0
+ 5021 × 800 which equals 4016800
+ 5021 × 7000 that is equal 35147000

= 39163800
