MYTUTOR SUBJECT ANSWERS

1044 views

How can I know whether to multiply or add probabilities?

We can only add probabilities when the events are alternatives. For example, let's say we want to calculate the probability that we achieve an A or a B or a C in a given exam. We cannot achieve an A and B for the same exam, so these events are indeed alternatives.

So let's say there's a 1/10 chance of getting an A, a 3/10 chance of getting a B and 4/10 chance of getting a C. The chance that we get either an A, B or C would be the sum of the individual probabilities: (1/10 + 3/10 + 4/10), so 8/10. 

Think about the problem numerically. We know that all probabilities lie in the range from 0 to 1. Addition of such numbers will lead to an increased probability value. Multiplication of such numbers will lead to a decreased probability value. In this case, we'd expect the probability of attaining A or B or C to be greater than the probability of simply attaining an A. And yes, we have an 8/10 chance of attaining an A,B or C and only a 1/10 chance of attaining an A - this is what we'd expect. 

We multiply probabilites when we want events to occur simultaneously, or consecutively. Let's say we're going to sit the same exam twice. We want to find the probability of attaining a C first time and an A on the re-sit. Before we dive in let's think about the expected probability value: getting a C AND THEN an A is a very unique scenario, and as such we'd expect quite a low probability in relation to the probabilities of individual events. This hints to us that multiplication could be the correct method.

So in this case, yes, we can say that the probability of getting a C and then an A is (4/10 * 1/10), so 1/25. 

Nathan O. GCSE Maths tutor, A Level Maths tutor, IB Maths tutor, GCSE...

2 years ago

Answered by Nathan, a GCSE Maths tutor with MyTutor


Still stuck? Get one-to-one help from a personally interviewed subject specialist

466 SUBJECT SPECIALISTS

£18 /hr

Abhinav T.

Degree: Medicine (Other) - University College London University

Subjects offered:Maths, Physics+ 2 more

Maths
Physics
Chemistry
Biology

“Confirming understanding by verbal confirmation and communication so as to ensure clarity and confidence during the learning process and progression.”

MyTutor guarantee

£18 /hr

Gabrielle F.

Degree: Philosophy, Politics and Economic (Bachelors) - Kings, London University

Subjects offered:Maths, English+ 2 more

Maths
English
Economics
.TSA. Oxford.

“Hi, I'm an enthusiastic tutor with experience in multiple subjects. I am currently studying a BSc in Philosophy, Politics and Economics at Kings College ”

£22 /hr

Haider M.

Degree: Medicine (Bachelors) - Exeter University

Subjects offered:Maths, Physics+ 7 more

Maths
Physics
Extended Project Qualification
Chemistry
Biology
.UKCAT.
.BMAT (BioMedical Admissions)
-Personal Statements-
-Medical School Preparation-

“A bit about me I am currently in my 2nd year of medical school at the University of Exeter. I really enjoy learning about science, especially the science of health disease, and would love nothing more than to pass down this knowledge ...”

About the author

Nathan O.

Currently unavailable: no new students

Degree: Computer Science (Bachelors) - Manchester University

Subjects offered:Maths, Physics

Maths
Physics

“Friendly Computer Science undergraduate at the University of Manchester. I'm here to help you with Maths and Physics at GCSE, A level or IB”

MyTutor guarantee

You may also like...

Other GCSE Maths questions

How to apply the quadratic equation

Mark has a voucher that gives him 22% off the prices at a hardware store. Estimate how much he will pay for an electric drill that normally costs £87.99. (non-calculator) (3)

Solve these simultaneous equations: y=3x-10; y=2x+5

How do I find the length of a side of a triangle using the cosine rule?

View GCSE Maths tutors

We use cookies to improve your site experience. By continuing to use this website, we'll assume that you're OK with this. Dismiss

mtw:mercury1:status:ok