1.

158+262+138 =158+(262+138) =158+400 =558

2.

375+219+381+225 =(375+225)+(219+381) =600+600 =1200

3.

5001-247-1021-232 =5001-(247+1021+232) =5001-1500 =3501

4.

(181+2564)+2719 =(181+2719)+2564 =5464

3+5+7=15

12+19+8=39

23+19+27=69

27+14+3=44

29+18+11=58

36+16+4=56

38+19+12=69

42+19+18=79。