`1 × 2 × 3 × 4 × … × 20`

`= (2 × 4 × 6 × …) × (5 × 10 × 15 × 20)` `\text{× các số còn lại}`

`\text{Có 4 cặp (2 × 5) = 10}`

`=>` `\text{tích có 4 chữ số 0 tận cùng}`

`=>` `4`

$\color{#FF4500}{\diamond\triangle\star}~\color{#FF0000}{ng}\color{#FF6600}{uy}\color{#FFCC00}{en}\color{#33CC33}{p}\color{#33CCFF}{h}\color{#6633FF}{uc}\color{#CC33FF}{th}\color{#FF3399}{an}\color{#FF3366}{g}\color{#FF0033}{26}\color{#FF6600}{04}~\color{#FF4500}{\star\triangle\diamond}$