Tính một cách hợp lí. a) –1,2 + (–0,8) + 0,25 + 5,75 – 2021

a)  –1,2 + (–0,8) + 0,25 + 5,75 – 2021

= [–1,2 + (–0,8)] + (0,25 + 5,75) – 2021

= –2 + 6 – 2021

= 4 – 2021

= –2017

b) $-\mathrm{0,1}+\frac{16}{9}+\mathrm{11,1}+\frac{-20}{9}$

$=\left(-\mathrm{0,1}+\mathrm{11,1}\right)+\left(\frac{16}{9}+\frac{-20}{9}\right)$

$=11+\frac{-4}{9}$

$=\frac{99}{9}+\frac{-4}{9}$

$=\frac{95}{9}$