Coin toss is nothing but the experiment of tossing a coin. When the probability of an event is same as zero, then the event is said to be impossible. When the probability is same as one then the event is said to be sure or certain. When we toss a coin we can only have two types of outcomes: heads or tails. At no point of time we can have both heads and tails as outcomes together whenever we flip a coin. We know that probabilities lie between 0 and 1. We always consider to have fair coins being tossed.

The sample space count can be decided by the number of coins being
flipped together. If we have flipped ‘n’ coins or flipped a coin ‘n’
times then the sample space in the experiment will have $2^n$ elements.

For
example in case of tossing a coin thrice we can have exactly 2^3 = 8
outcomes that are HHH, HHT, HTH, THH, TTH, THT, HTT, TTT.

We
know that probability of an event is the ratio of the number of
outcomes associated with that event to the total number of outcomes of
the experiment. Here, suppose we need to find the probability of getting
all heads, then there is only one outcome possible in such a case so
the probability of this event is 1/8.

Also we see that at any
point of time both heads and tails cannot come as outcome
simultaneously. These events have an equally likely chance and also
their probability is same in the experiment of tossing a coin.

Thus
the events of getting a head / a tail are mutually exclusive. It is
impossible to get a head and a tail simultaneously when a coin is
flipped and the event of getting either a head or a tail is a certain
event.

When we add the probability of getting a
head and probability of getting a tail when a coin is tossed the sum is
always equal to 1.

In general when we have
flipped a coin large number of times then we are making a relative
frequency estimate of probability on easy basis. For example if we
calculate the probability of getting 75 heads out of 100 times then we
say it will be $\frac{75}{100}$ = 0.75. as we flip the coin more number
of times more approximation we get.

This means
rather getting 0.5 directly we will get answers approaching to 0.5 or
other values. For example if we have flipped a coin 10000 times and we
got 5678 times heads, then the probability is 0.5678 which is not
exactly 0.6. It is simply approaching to 0.6.

Let the number of coins be ‘n’.

We know that $2^n$ = 128

$\rightarrow$ $2^n$ = $2^7$

$\rightarrow$ n = 7

Hence, 7 coins are flipped together.

S = {TTT, THT, TTH, HTT, THH, HTH, HHH, HHT}

$\rightarrow$ N (S) = 8

B = At least 2 tails = {TTT, TTH, THT, HTT}

$\rightarrow$ N (B) = 4

P (B) = $\frac{4}{8}$ = $\frac{1}{2}$