a ) `76 + 78 + 80 - 70 - 68 - 66`
`= ( 76 - 66 ) + ( 80 - 70 ) - ( 78 - 68 )`
`= 10 + 10 + 10 = 30`
b ) `1 + 2 + 3 + 4 + ... + 17 + 18`
`= 1 + ( 2 + 18 ) + ( 3 + 17 ) +...+ ( 9 + 11 ) + 10`
`= 11 + ( 20 xx 8 )`
`= 11 + 160 = 171`
c ) `6 xx 7 + 12 xx 6 + 6 xx 81`
`= 42 + 72 + 486 = 600`