a)815-23-77+185
=815-(23+77)+185
=815-100+185
=715+185
=900
b)3145 + 2496 + 5347 + 7504 + 4653
=(2496+ 7504)+(4653+5347)+3145
=10000+10000+3145
=23145
c)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
=100
d) 52 - 42 + 37 + 28 - 38 + 63
=(52+28)+(37+63)-(42+38)
=80+100-80
=180-80
=100
xin ctlhn ạ ít điểm quá