Đáp án:
`(2021 × 13 + 2007 + 2020 × 2007)/(2020+2020×520+1500×2020)`
`= (2021 × 13 + 2007 × 1 + 2020 × 2007)/(2020 × 1 + 2020 + 520 + 1500 × 2020)`
`= (2021 × 13 + 2007 × (1 + 2020))/(2020×(1 + 520 + 1500))`
`= (2021 × 13 + 2007 × 2021)/(2020 × 2021)`
`= (2021 × (13 + 2007))/(2020 × 2021)`
`= (2021 × 2020)/(2020×2021)`
`= 1`