a, 1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 + 19

= ( 1 + 19 )+( 3 + 17 )+( 5 + 15 )+ ( 7 + 13 )+ ( 9 + 11 )

= 20 + 20 + 20 + 20 + 20

= 20 × 5

= 100

b, 15 × 25 + 25 × 34 + 51 × 25

= 25 × ( 15 + 34 + 51 )

= 25 × 100

= 2 500

a, 1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 + 19

= ( 1 + 19 )+( 3 + 17 )+( 5 + 15 )+ ( 7 + 13 )+ ( 9 + 11 )

= 20 + 20 + 20 + 20 + 20

= 20 × 5

= 100

b, 15 × 25 + 25 × 34 + 51 × 25

= 25 × ( 15 + 34 + 51 )

= 25 × 100

= 2 500

a, 1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 + 19

= ( 1 + 19 )+( 3 + 17 )+( 5 + 15 )+ ( 7 + 13 )+ ( 9 + 11 )

= 20 + 20 + 20 + 20 + 20

= 20 × 5

= 100

b, 15 × 25 + 25 × 34 + 51 × 25

= 25 × ( 15 + 34 + 51 )

= 25 × 100

= 2 500

a, 38 569 034

+

70 728129

____________

109 297 163

b,

3 245 762.

_

1 053 881

____________

2 191 881

C,

13 719

×

25

_______________

55 595

27 435

_________________

32 9 945

3 384 ÷ 36 = 94

205 336 + 1 536 ÷ 24 = 1 536÷24

=205 336 + 64

=205 400

750 000+2 325×4-50 30

=

=

3 phút 20 giây = 200 giây

2 ạ 54 kg > 245kg

105 yến = 1 tấn 50 yến

1 thế kỉ < 1 000 năm