#### Answer

$4.05\times10^{-8}$

#### 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.
To convert $0.0000000405$, we have to move the decimal place $8$ places to the right, giving us a coefficient of $4.05$ and making the power of $10$ be $-8$.
This leaves us with the expression: $4.05\times10^{-8}$