MISSOULA, MTFor the seventh week in a row, Bill Trusky cheated his son Shane out of the boy's $3 allowance, the 8-year-old's father said Monday. 'Sorry, Shane, I said it was double or nothing if you could sneeze with your eyes open,' Trusky said. 'But I'll tell you what: If you can mow the lawnfront and backin 20 minutes, I'll pay you triple.' Household sources report that Shane might have completed the task had Trusky not hurled a croquet ball in the mower's path 10 feet before his son finished. 