In a class of 50 students,30 study Biology and 25 study Economics. 15 study neither Biology nor Economics. (A) How many take both subjects? (B) How many take Biology only? (C) How many take Economics only?

Answer 1

Well... In total they have to all add up to 50 (that is the ones who take biology, economics, both bio and econ, and those that take neither bio nor econ).
So...
If you draw two intersecting circles in your venn diagram. We see the Universal set is 50 (that is there are 50 kids in total).
Now... for all of the parts inside the universal set to add up to 50 there must be 20 that take both bio and econ, that leave 30-20 = 10 that take bio only, so following the same logic (taking the intersection away from the econ total) you will have those that take econ only.
Note: 20 had to be the intersection (trial and error) because that was the only way the parts and pieces added up to the universal set (50) : )

Answer 2

50 students,30 study Biology and 25 study Economics. 15 study neither
so, 50-15 = 35 study one or the other or both. If x study both, then
30+25 - x = 35
x = 20

Answer 3

Prefect answer