We write a number in the form abcdef.ghijkl as a.bcdefghijkl multiplied by an appropriate power of 10. In this example, we moved the decimal point 5 times to the left, so we would have a.bcdefghijkl x 10^5.
Here, to do the same we need to move the decimal point to the right, so we are essentially dividing by a power of 10, or multiplying by a negative power of 10, and the decimal point needs to be moved 4 positions:
0.0002405 = 2.405 x 10^(-4)