①∵1km=103m，∴35km=35×103m=3.5×104m；②∵1mm=10-3m，∴0.2mm=0.2×10-3m=2×10-4m；③∵1h=3600s，∴0.5h=0.5×3600s=1800s．故答案为：3.5×104；2×10-4；1800．
