My doubt is on the following question from 2013 board paper:

Q: Assume that the chances of a patient having a heart attack is 40%. Assuming that a meditation and yoga course reduces the risk of heart attack by 30% and prescription of certain drug reduces its chance by 25%. At a time a patient can choose any one of the two options with equal probabilities. It is given that after going through one of the two options, the patient selected at random suffers a heart attack. Find the probability that the patient followed a course of meditation and yoga. Interpret the result and state which of the above stated methods is more beneficial for the patient.

The answer given to this board question was:

A: Let A, E1, and E2, respectively denote the events that a person has a heart attack, the selected person followed the course of yoga and meditation, and the person adopted the drug prescription.

Probability that the patient suffering a heart attack followed a course of meditation and yoga =

Now, calculate P(E2|A).

End of Answer~~

Now i come to my question. It says that A is the event the person has a heart attack, E1 is the event that he takes meditation course and E2 is the event that he takes drugs. Now, the statement "P(A|E1) = 70/100 * 40/100." is confusing since P(A|E1) is supposed to be event that the person suffers a heartattack given that he has taken yoga and meditation.

Now, as per the question, Yoga and Meditation will reduce the risk of heart attack by 30%, which means 30% of 40% (where 40% is the risk of person having a heart attack) = 12% , should be his current risk of heart attack (after doing yoga and meditation). Therefore P(A|E1) should be 12% (i.e 0.12), Am i not right? How did they take it to be 70/100 * 40/100? Please give a detailed explanation. Thanks in advance.

(Experts please help, I have unit test coming up soon).

Here A is the event of person prone to heart attack.
It does not mean that they have suffered from heart attack.
And P(A/E1 )  is the probability of person suffered from heart attack , after using meditation and if the meditation succeded then there is no chance of heart attack, but it failed , so there is 70 percent chance of failure.

Here the key thing is person suffered from heart attack. If meditation works then he will not suffer from heart attack, but it fails, so you have to use 70%

Hope you have got it.

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