Clep math probability add or multiply ? whats the trick?

you know those tricky probability questions some times you have to add and some times multiply I d like to know if some one has a trick to recognize when to do so? another question is what to do when you see ''OR''? looks like its another trick


here 's a question from college board official math test sample :





In a Class, with 50 students,25 are female. 15 are math majors,10 of math majors are female,If a student is to be selected at random, what is the probability that the student selected will be female OR math major OR both?


A-0.4 B-0.5 C-0.6 D-0.8

Clep math probability add or multiply ? whats the trick?
Drawing ellipses, you will have one with 25 females overlapping math majors, with 10 in the union, and 15 outside the union in the math majors. Also, draw another ellipse for the males, with 25 members overlapping 5 in the math major's ellipse.





The probability of the chosen student being a female is 25/50 or .5. The probability of the student being a math major is 15/50 or .3. The probability of being both is the overlapping 10/50 or .2. Also, you must subtract the probability of being a male that is not a math major which is 20/50 or .4.





The answer will be .5 + .3 + .2 - .4 = .6 or 60%.

flowering plum