@Active Activity.
$a)$ $21 + 68 +279 + 132$
$=$ $(21+279)+(68+132)$
$=$ $300+100$
$=$ $400$
$b)$ $83×72+28×83$
$=$ $(72+28)×83$
$=$ $100×83$
$=$ $8300$
$c)$ $35×17+84×35-35$
$=$ $(17+84-1)$
$=$ $35×100$
$=$ $3500$
$d)$ $62,87+35,14+4,13-8,35+4,86-5,65$
$=$ $(62,87+4,13)+(35,14+4,86)+(8,35+5,65)$
$=$ $67+40+14$
$=$ $121$
$e)$ $3,54×73+0,32×25+3,54×27+0,17×25$$
$=$ $3,54×(73+27)+25×(0,23+0,17)$
$=$ $3,54×100+25×0,4$
$=$ $354+10$
$=$ $364$
$f)$ và $g)$: Phần này mình chịu ><