a) 234456 - 123798
= 110658 + 123798 - 123798
= 110658
b) 643546 - 237626 - 132004
= 273916 + 237626 + 132004 - 237626 - 132004
= 273916
c) 102078 - 98968 + 120330
= 3110 + 98968 - 98968 - 3110 + 123440
= 3110 - 3110 + 123440
= 123440
d) 247122 + 6929 - 230400
= 23651 - 6929 + 230400 + 6929 - 230400
= 23651