a) 1 267 + 99 + 501 = 1 267 + (99 + 501)

                                = 1 267 + 600 = 1 867

     25 x 14 x 4 = (25 x 4) x 14

                       = 100 x 14 = 1 400

b) 3 905 x (50 – 1) = 3 905 x 50 – 3 905 x 1

                             = 195 250 – 3 905 = 191 345

     270 : ( 27 x 2 ) = 270 : 27 : 2

                             = 10 : 2 = 5

c) 115 x 58 + 115 x 42 = 115 x (58 + 42)

                                   = 115 x 100 = 11 500

136 x 67 – 136 x 66 = 136 x (67 – 66)

                              = 136 x 1 = 136

28600

68×171+115×101+171×32-115

=(68 + 32) x 171 + (101 - 1) x 115

=100 x 171 + 100 x 115 

=17100 + 11500= 28600

b) 168 + 2080 + 32= (168 + 32) + 2080

= 200 + 2080 = 2280

87 + 94 + 13 + 6= ( 87 + 13) + (94 +6)

= 100 + 100

121 + 85 + 115 +469= (121 + 469) + ( 85 + 115)

= 590 + 200 = 790.

(115+125) + (135 + 145) + 160

= 240 + 280 + 160

= (240+160) + 280

= 400 + 280

= 680

115+125) + (135 + 145) + 160

= 240 + 280 + 160

= (240+160) + 280

= 400 + 280

= 680

168 + 2080+ 32

= ( 168 + 32 ) + 2080

= 200 + 2080

= 2280.

87 + 94 + 13 + 6

= ( 87 + 13 ) + ( 94 + 6 )

= 100 + 100

= 200.

121 + 85 + 115 + 469

= ( 121 + 469 ) + ( 85 + 115 )

= 590 + 200

= 790.

a, 168+2080+32

=(168+32)+2080

=200+2080

=2280

b,87+94+13+6

=(87+13)+(94+6)

=100+100

=200

c,121+85+115+469

=(121+469)+(85+115)

=590+200

=790

= ( 11 + 89 ) + ( 22 + 78 ) + ( 33 + 67 ) + ( 44 + 56 ) + 55

= 100 + 100 + 100 + 100 + 55

= 400 + 55

= 455

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=<89+11>+<78+22>+<67+33>+<56+44>+55

=100+100+100+100+55

=400+55

=455

Bài 5:

a: 2x=10

nên x=5

b: x:6=5

nên x=30

Hướng dẫn giải:

a) 93 + 39 + 7 = (93 + 7) + 39 = 100 + 39 = 139.

89 + 85 + 11 = (89 + 11) + 85 = 100 + 85 = 185

184 + 66 + 16 = (184 + 16) + 66 = 200 + 66 = 266

b) 23 + 52 + 47 + 98 = (23 + 47) + (52 + 98) = 70 + 150 = 220

16 + 490 + 84 + 10 = (16 + 84) + (490 + 10) = 100 + 500 = 600 

\(1235\text{×}6789\text{×}\left(630-315\text{×}2\right)\)

\(=1235\text{×}6789\text{×}\left(630-630\right)\)

\(=1235\text{×}6789\text{×}0=0\)

\(\left(100+67\right)\text{×}67+\left(200-38\right)\text{×}33\)

\(=167\text{×}67+167\text{×}33\)

\(=167\text{×}\left(67+33\right)\)

\(=167\text{×}100=16700\)

\(72+36\text{×}2+24\text{×}3+18\text{×}4+12\text{×}2+168\)

\(=72+72+72+72+24+168\)

\(=\left(72\text{×}4\right)+24+168=288+24+168=480\)

a) \(1235\times6789\times\left(630-630\right)\)

\(=1235\times6789\times0\)

\(=1235\times0\)

\(=0\)

b) \(167\times67+167\times33\)

\(=167\times\left(67+33\right)\)

\(=167\times100\)

=16700