Estimating with scientific notation

To estimate, round each decimal part to the nearest whole number, then do the arithmetic. For (2.3 × 10⁻⁴)(6.4 × 10¹²)4.2 × 10¹⁰: round to (2 × 10⁻⁴)(6 × 10¹²)4 × 10¹⁰ = 12 × 10⁸4 × 10¹⁰ = 3 × 10⁻². The exact answer is about 3.50 × 10⁻², so the estimate is close.

It works for ordinary numbers too: write each one in scientific notation first, round, and multiply.