`(m + 20,21) + (m + 20,21) + (m + 20,21) + (n + 20,21) + (n + 20,21) + (n + 20,21)`
`= 3 × (m + 20,21) + 3 × (n + 20,21)`
`= 3 × (m + 20,21 + n + 20,21)`
`Thay m = 20,21 - n`
`= 3 × (20,21 - n + 20,21 + n + 20,21)`
`= 3 × (20,21 + 20,21 + 20,21 - n + n)`
`= 3 × (20,21 + 20,21 + 20,21)`
`= 3 × 60,63`
`= 181,89`