Explanation

We will use the general rule of addition by making the powers same. $ie,$ we will add $3.8 \times 10^{- 6}$ and $42 \times 10^{- 6}$ we get $\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 45.8 \times 10^{- 6} = 4.58 \times 10^{- 5}$ As least number of significant figures in given values are 2, so We round off the result to $4.6 \times 10^{- 5}.$