How long is a distance of 8 km if measured on a map with a scale of 1: 50,000

Since this is a ratios and proportions problem, you would set up the equation like this:
1/50,000  = x/8                      x is the length of the distance on the map

After that, you cross multiply and you will get 50,000x=8  
You then solve for x to get 0.00016