DipikaP wrote:

For those who have trouble grasping the concept behind this - check out Khan Academy lessons on probability and combinatorics. I'm not allowed to post urls as a newbie but a simple Google search will throw up the link.

I couldn't make head or tail of these questions before seeing them, tried memorizing the formulae and always messed up. I invested 3 hours in going through those videos and can now solve these questions without knowing any formulae - it's all conceptual. Sal's great with breaking down concepts to simple, relatable stuff.

Hi Dipika,

You are totally right when you say "it is all conceptual". Trying to memorise formulae is not the right approach.

For example in this question:

First we should think what are the possible outcomes when we toss a coin: head or tail (2 outcomes)

Now as the coin is fair, the probability that we will get a head or a tail is 1/2

To illustrate, let's take a smaller version of the above question:

What is the probability of getting more heads in 3 tosses?1st case: We can get 3 heads: HHH

Probability of HHH = 1/2 * 1/2 * 1/2 = 1/8

Probability of getting 3 heads = 1/8

2nd case: We can get two heads: HHT, HTH, THH

Probability of HHT = 1/2 * 1/2 * 1/2 = 1/8

Probability of HTH = 1/2 * 1/2 * 1/2 = 1/8

Probability of THH = 1/2 * 1/2 * 1/2 = 1/8

Probability of getting 2 heads = 3* 1/8

As you can see HHT, HTH and THH are different arrangements of HHT

Probability of getting 2 heads = (No. of arrangements of HHT)* (Probability of getting HHT) = 3!/2! * (1/2 * 1/2 * 1/2) = 3/8

Total probability of getting more heads in 3 tosses = 1/8 + 3/8 = 4/8 = 1/2

Thinking on these lines you can solve the above question easily.

But if you think a little further, we are talking about odd number of tosses (3 and 7). So, either there will be more heads or more tails. It is not possible to get equal number of heads and tails. Hence, in half of the outcomes we will get more heads than tails and in the the other half we will have more tails than heads. Thus, the probability of getting more heads = probability of getting more tails = 1/2.