Tính nhanh: a) 42 + 257 + 43+158; b) 205 + ( 2003 + 95); c) 283+119

a) 42 + 257 + 43+158

= (42 + 158) + (257+43)

= 200 + 300

= 500

b) 205 + ( 2003 + 95)

= (205 + 95) + 2003

= 2303

c) 283+119 + 37+17 + 81

= ( 283 + 17) + (119 + 81) + 37

= 300 + 200 + 37

= 537

d) (25.35).40

= (25.40).35

= 1000.35

= 35000

e) (125.9).80;

= (125.80).9

= 90000

f) 11 + 12 + 13 + …+ 28 + 29

= (11+29) + (12+28) + ....

= 19.(11 + 29):2 

= 19.40:2

= 380