Answer
$4.2\times10^{-3}$
Work Step by Step
To convert a number from standard notation to scientific notation, we must move the decimal to the left or to the right, counting the number of places, until the number is between $1$ and $10$, and multiply by a power of $10$. If the number is less than $1$, and we moved the decimal to the right, the power of $10$ is the negative number of places we moved the decimal point. If the number is greater than $1$, and we moved the decimal to the left, the power of $10$ is the positive number of places we moved the decimal point.
When trying to convert $0.0042$ to scientific notation, we have to move the decimal point to the right $3$ places. This gives us a coefficient of $4.2$, and our power of $10$ will be $-3$.
This gives us the expression: $4.2\times10^{-3}$
