`a)`
`(279-1987)+(-18+1987-279)`
`= 279 - 1987 -18+1987 - 197`
`= (279-279)+(1987-1987)-18`
`=0+0-18`
`=-18`
`b)`
`-(3251+415)-(2000+585-251)`
`= -3251-415-2000-585+251`
`= (-3251 + 251) -(415+585)-2000`
`= -3000 - 1000 - 2000`
`=-6000`
`c)-25-26-27-28-29+15+16+17+18+19`
`= (-25+15)+(-26+16)+(-27+17)+(-28+18)+(-29+19)`
`= -10-10-10-10-10`
`=-50`
`d)`
`71-(-30)-18+(-30)+118`
`= 71 + 30 -18 - 30`
`= (30-30) + (118-18) + 71`
`=0+ 100 + 71`
`= 171`