Break 8000 into 1000 × 8, then divide step by step.<br><br>
&bull; 1680 ÷ 8000 = 1680 ÷ (1000 × 8) = 1680 ÷ 1000 ÷ 8<br>
&bull; 1680 ÷ 1000 = 1.68<br>
&bull; 1.68 ÷ 8 = 0.21<br><br>
Final Answer: First blank = 1000, second blank = 1.68, third blank = 8, fourth blank = 0.21