Math probability question! Please help!!!?

Question

In a first year Calculus class, 40% of the students have taken high school Calculus. The pass rate in first year calculus for those students is 90%. The pass rate for those students who did not take high school Calculus is 50%. If a student passes University Calculus, what is the probability that the student did take high school Calculus??

Please try to explain your answer if possible! I need to learn this :)

Answer 1

Say there are 100 students in the class
40 took high school calc
60 didn't

90% of the 40 passed
50% of the 60 passed

so
36 people took high school calc and passed
+30 people didn't but still passed
66 people total passed

so what are the odds that someone who passed took high school calc?

36/66 = .55

Sometimes using real numbers can make it easier to see what's going on. Good luck

Answer 2

http://math.about.com/od/probability/Probability.htm

Let's say there are 100 students in the class.
40 took HS calc. 90% of those pass, gives you 36 students.
60 did not take HS calc. 50% passed = 30 students.
66 students (36 + 30) passed altogether.

Probability = 36/(36+30) = 0.54545..
0.545 x 100 = 54.5% PROBABILITY that they took HS Calc.

Make an example - draw a picture. Make a hypothetical case - 100 students.
Numbers 100 or 1000 are easy to work with.

Answer 3

(0.4x0.9) + (0.6x0.5): all goes in the denominator. this is the probability of people who pas overall. The first set of brackets is for the amount of people who will pass having taken high school calc. the second bracket is for those who will pass not having taken high school calc.

in the numerator you put (0.6x0.5) for people in the class who will pass not having taken high school calc. Then you should get a fraction or decimal number (less than 1). Hope that helps! not too sure if its right... just a hunch!

Answer 4

Nick's approach is good but not sure it answers the question, I think it answers a different question so check your question vs his answer