First, the total number of people is represented as 1.
Or, the number of people (in fraction) who ate tacos, hamburgers and french fries, and hamburgers only, add up to 1.
Since 1/5 of the people ate tacos, and 3/7 of the people ate both hamburgers and fries, we get the number of people who ate hamburgers only is :
1 - 1/5 - 3/7 = 13/35 (or 13 people).