Why are SI prefixes needed? Rewrite 6.4 × 10⁻⁶ m and 2.5 × 10⁷ W with suitable prefixes, and state two rules for using prefixes.

1. 1 · Why prefixes. Very large or very small values have many zeros; prefixes give short, easy numbers.
2. 2 · First value. 6.4 × 10⁻⁶ m: 10⁻⁶ is micro, so it is 6.4 μm.
3. 3 · Second value. 2.5 × 10⁷ W = 25 × 10⁶ W = 25 MW (mega = 10⁶).
4. 4 · Rule one. Choose the prefix that keeps the number between 0.1 and 1000.
5. 5 · Rule two. No space between prefix and unit symbol, and only one prefix at a time.
6. 6 · Close. So prefixes make values short and clear.

Scoring tip: Write the power of ten you used before the prefix; it shows your working.