Problem:

In a class of 40 students, 17 have ridden an airplane, 28 have ridden a boat. 10 have ridden a train, 12 have ridden both an airplane and a boat. 3 have ridden a train only and 4 have ridden an airplane only. Some students in the class have not ridden any of the three modes of transportation and an equal number have taken all three.

Let Sets: A = airplane, B= Boat, T = train

I understand some of the number of students to be on the parts of the circles intersecting... there is a little bit confusing on the example given and the solution also... i tried to fix it on (A intersection B )

in the (A intersection B) = the number of students has ridden A and B...

my problem is how it was being separated in to 8 and 4... i could not get the rationale over that part...

thanks for your help mathematicians..