796 is the quantity (number of) people and is an integer. 5% of 796 is 796/20=39.8, but in absolute terms we need a quantity of people, so we use rounding to change 39.8 to its nearest integer, which is 40. 40 is closer than 39, so 40 is taken to be the nearest integer. In absolute terms the survey determines that 796±40 people represents the results of the survey. In other words, the range of people satisfying the survey is between about 796-40=756 and 796+40=836. This is an example of converting a percentage into an actual number (a whole number or integer) which, in this context, makes more realistic sense than using the decimal containing a fraction. Expressing the margin of error as a percentage is just a convenient way and, from a statistical point of view, is more efficient than using actual absolute values, because it opens the way to compare the results of different surveys involving different numbers of people. For example, two other surveys could consist of 500 people or 2300 people. Let's say the range of people satisfying each survey is 475-525 and 2185-2415 respectively. In each case the actual range of numbers of people is changed to a percentage which happens to be 5% for each case. For comparison purposes, then, we can see that the results of the surveys are the same if we use percentage instead of actual numbers.
