logarithms: light intensity word question

[coyotes]

"In a particularly murky river, the intensity of light is reduced by 5% for every metre that the light travels below the surface of the water."
To the nearest metre, at what depth would there be only 10% of the original intensity of the light remaining?

I don't even know where to start with this. I know that the exponential model is L(d) = (.95)^d, if that helps.

 

[Deleted member 4993]

"In a particularly murky river, the intensity of light is reduced by 5% for every metre that the light travels below the surface of the water."
To the nearest metre, at what depth would there be only 10% of the original intensity of the light remaining?

I don't even know where to start with this. I know that the exponential model is L(d) = (.95)^d, if that helps.

Is the equation:

L(d) = (.95)^d

or,

are you supposed to derive it?