40/84 as a percentage is 47.62%. This number tells you how much of a whole the fraction represents when you look at it in terms of hundredths.
The fraction 40/84 can be reduced to its simplest form, which is 20/42, and then further to 10/21. Even in this reduced state, it still represents the same portion of a whole, just expressed with smaller numbers.
Knowing the percentage helps when you need to compare this part to other values that are already in percent form. For example, if you have a test scored out of 100 points, a score of 47.62% would be equivalent to getting 40 out of 84 possible points.
