a) (-5)-(-4)-(+5)-(-2)-(+7)=
=-5+4-5+2-7
=-1-5+2-7
=-6+2-7
=-4-7
=-11
b) (-21+11)-(50-54)=
=-10-(-4)
=-10+4
=-6
c) (-5-7+8)-(-10+15-25)=
=(-12+8)-(+5-25)
=-4-(-20)
=-4+20
=16
d) (-16+12-25+15)+(-7+5)-(-16+15)=
=(-4-25+15)+(-2)-(-1)
=(-29+15)-2+1
=-14-2+1
=-16+1
=-15