We have a lamp and it is in 2m distance. Its light intensity is 5. We want to know what amount of light is reaching to us. We can simply use a formula like: $ \mathrm{light} = \frac{\mathrm{intensity}}{\mathrm{distance}} $.
Now suppose that the distance is a random variable (i.e. it is in $ 2m \pm 1m $ distance, and the intensity is $ 5 \pm 4.7 $).
How we can change the formula in order to be able to use it in case we have random variables too?
P.S. Actually the question is how we can make use of probability distributions, variance, etc. to change the formula?