Bài 1:
a,647+238+533+762+3465=(647+533)+(238+762)+3465
=1180+1000+3465
=5645
b,1234+5678+4321+8765=6912+13086
=19998
Bài 2:
a,x-105:5=21
x-21 =21
x =21+21
x =42
b,(x-105):5=21
(x-105)=21.5
x-105=105
x=105+105
x=210
c,(x-48):2-4=21
(x-48):2=21+4
(x-48):2=25
x-48=25.2
x-48=50
x=50+48
x=98
d,(x+321):8-125=25
(x+321):8=25+125
(x+321):8=150
x+321=150.8
x+321=1200
x=1200-321
x=879
e,(12x-144)-240=360
(12x-144)=360+240
12x-144=600
12x=600+144
12x=744
x=744:12
x=62
CHÚC BẠN HỌC TỐT>_<!