1. Tính tổng một cách hợp lý

117 + 68 + 23

=117+23+68

 =140+68

 =208

127+39+73 

 =127+73+39

 =200+39

 =239

135+360+65+40

=135+65+360+40

 =200+400

 =600

 285+470+115+230

=285+115+470+230 

 =400+700=1100

 571+216+129+124

=571+129+216+124

=700+400

=1100

21+22+23+24+25+26+27+28+29+30

=21+29+22+28+23+27+24+26+20+30

=50+50+50+50+50

2021+2022+2023+2024+2025+2026+2027+2028+2029

=2021+2029+2022+2028+2023+2027+2024+2026+2025

=5000+5000+5000+5000+2500 =22500

 1+2+3+4+...+97+98+99+100

 =1+100+2+99+3+98+...+49+52+50+51

  =101+101+101+...+101+101⏟50​  soá   

=101. 50=5050

 2+4+6+8+...+96+98+100

 =2+98+4+96+6+94+...+48+52+50+100

 =100+100+100+...+100⏟24  soá+150

  =100 . 24+150=2550

15+17+19+21+...+73+75+77 

 =15+75+17+73+...+43+47+45+77

= 90+90+...+90+90⏟15 soá+122

 =90 . 15 +122= 1472

97+ 19= 97 + 3 + 16= 100 + 16=116

996 + 45= 996 + 4 +41=  1000 + 41=1041

37 + 198= 35 + 2 + 198= 35  + 200=235

45 + 296=  41 +  4 + 296=  41 + 300=341

488  +  66=   488  + 12   + 54=​​ 500  + 54=554