#### Answer

$1.234\times10^{5}$

#### 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 then we 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.
For the number $123,400$, we move the decimal to the left $5$ places, giving us a coefficient of $1.234$ and a power of $10$ to the positive $5$. In scientific notation, this is $1.234\times10^{5}$.