Look at this problem in terms of a generic problem, asking, "What percent of 10 is 4" and that would be x% * 10 = 4. 4 divided by 10 is .4, and since a percent is expressed by multiplying a decimal by 100, you get that 4 kilometers is 40% of 10 kilometers. Which makes sense because 5 kilometers would be half of the 10 kilometers, which would be 50%...
