The nation's unemployment rate dipped in March to 5 percent, the lowest rate in more than 15 years, despite a slowed rate in the creation of jobs, the Labor Department said Friday. (Utah's rate rose to 4.1 percent; see A3 for details.) More than 180,000 new jobs were added to non-farm payrolls last month, the lowest figure since 154,000 in August 1988 and down from a revised February figure of 280,000. The slower rate of job creation, affected somewhat by the Eastern Airlines strike, indicates some slowdown in the economy. But there were signs of continued strength: The number of factory jobs remained stable, and the average factory work week fell slightly, by 0.2 hours to 40.9 hours. The 5 percent rate - the lowest since December 1973 when it was 4.9 percent - defied predictions that unemployment would edge up slightly.