Here's what they are trying to get at. 48 people took the exam and they want to know how many people passed at least one subject. So the bare minimum would be 33, because that's how many people passed math.
The rest of the information is to tell you how many people passed physics and french, but did not pass math. If 21 people passed all 3 subjects that means that
33 - 21 = 12 people did not pass all 3, but passed math
32 - 21 = 11 people did not pass all 3, but passed french
27 - 21 = 6 people did not pass all 3, but passed physics
if 26 people passed physics and math, that means that 1 person, out of the 27 that passed physics did not pass math. So the number of people who passed at least one subject is now 34.
if 26 people passed french and math, that means that 6 people passed french but not math, which brings the total to 40.
40 is the amount of people who passed at least 1 test.