MINEOLA — Nassau Executive Edward Mangano announced Wednesday he is taking steps to fix the county's tax assessment system. Mangano pointed out that half of Nassau's $3 billion debt is associated with tax grievances. The county executive said 90,000 homeowners out of 160,000 who grieved their tax bills saw a reduction. As a result of the reforms, the county is now operating on a four-year reassessment system with re-evaluations that will begin in 2013 for the 2014-2015 tax year.