Question 7 Evaluate using scientific notation; with the decimal part rounded to three significant digits when necessary. \[ \left(5.4 \times 10^{-6}\right)\left(1.8 \times 10^{-6}\right)\left(4.2 \times 10^{7}\right) \] $4.082 \times 10^{3}$ $4.08 \times 10^{-3}$ $4.082 \times 10^{4}$

Step 1 ：Multiply the numbers: \(5.4 \times 1.8 \times 4.2 = 40.824\)

Step 2 ：Add the exponents: \(-6 + -6 + 7 = -5\)

Step 3 ：Convert to scientific notation: \(40.824 \times 10^{-5} = 4.0824 \times 10^{-4}\)

Step 4 ：Round to three significant digits: \(4.0824 \times 10^{-4} = 4.082 \times 10^{-4}\)

Step 5 ：\(\boxed{4.082 \times 10^{-4}}\)