解答：解：作EF∥AB．∵AB∥CD，∴EF∥CD．∴∠A+∠AEF=180°，∠C+∠CEF=180°．∵∠A=123°，∠C=147°，∴∠AEF=57°，∠CEF=33°，∴∠AEC=57°+33°=90°．故答案为90．
