PROBABILITY OF GRE
Probability is one of these grab bag Quant areas that we 're going to see occasionally
occasionally. Probability. One of those things you 're reliably see anywhere from two to four
questions on the test. We 'll work through some increasingly difficult problems with probability
problems to work out how to do these probability problems.. Probability is always going to be
between 0 and 1. The most it could be is one what 's the probability I roll a number okay six out
of six so let 's try this out two more questions here. Uh see how you can do on that and I'll just
put that definition up on the board here successes over total. 5. 5 is my Center here and let 's
just deal with this before we think about other cases what 's the probability that the number
selected is greater than the average. be. The average of these middle two numbers and I 'm
giving a lot of like uh stats stuff here that maybe feels unfamiliar if it does that's okay..
The best thing you can do as a test taker is to focus on what you actually need that 's going to
help you break the problem down into pieces and make it actually possible to solve it so so far.
What we 've been doing is dealing with the probability of one thing happening. I roll the dice.
Once I roll this 24-sided Cube once we pick one number from a set of ten. I 'm going to change
it a little bit we're now going to roll our dice twice so the term we would use in probability talk is
to say we 're now dealing with two events. Roll one and roll two. When We have multiple events.
One of the things we need to be looking out for is there multiple success scenarios. In other
words, where I get a prime and non--prime. within each scenario. With multiplication it 's only
when we 're saying this scenario or this scenario that we add them together. That distinction can
sometimes be a little confusing. At first. We 're looking for the probability that we get a heads
and a six so it 's one half times one-sixth which is 1 over 12 and our answer is B good hopefully.
This is becoming a little clearer. Let 's keep going we 're going to start upping the difficulty a little
bit okay so probability for any given day is 1 6 to have rain..
The number of the chips do not matter no it totally does it matters that there's 12 of them and
then 11 of them. The threeness of it specifically matters no more than rolling a three on dice, so
as we 're doing these probabilities with multiple events we need to ask ourselves are we dealing
with replacement like rolling dice or flipping coins where the probability is constant. The
probabilities are n't going to change. Let 's try another one again. We 're rolling this dice twice.
We want the probability at least one of the rolls is greater than four so let 's think about the ways
this could happen. If either the first is more than four and the second is less than four. The
probability of each of these individually is going to be 1-6 times 1 6 for 1 over 36. The Other
solution. would be if we see this five success scenarios? We can ask how many total cases and
for that we would need to use combinatorics.. When We 've got multiple success scenarios and
you see something like at least or at most you want to ask yourself, which is easier successes
or failures.. The easier method is to do one minus failure because there are fewer failure
scenarios so that 's all the time we've got for right now. I 'm happy to answer any questions you
might have about the [UNK] or anything else..
Probability is one of these grab bag Quant areas that we 're going to see occasionally
occasionally. Probability. One of those things you 're reliably see anywhere from two to four
questions on the test. We 'll work through some increasingly difficult problems with probability
problems to work out how to do these probability problems.. Probability is always going to be
between 0 and 1. The most it could be is one what 's the probability I roll a number okay six out
of six so let 's try this out two more questions here. Uh see how you can do on that and I'll just
put that definition up on the board here successes over total. 5. 5 is my Center here and let 's
just deal with this before we think about other cases what 's the probability that the number
selected is greater than the average. be. The average of these middle two numbers and I 'm
giving a lot of like uh stats stuff here that maybe feels unfamiliar if it does that's okay..
The best thing you can do as a test taker is to focus on what you actually need that 's going to
help you break the problem down into pieces and make it actually possible to solve it so so far.
What we 've been doing is dealing with the probability of one thing happening. I roll the dice.
Once I roll this 24-sided Cube once we pick one number from a set of ten. I 'm going to change
it a little bit we're now going to roll our dice twice so the term we would use in probability talk is
to say we 're now dealing with two events. Roll one and roll two. When We have multiple events.
One of the things we need to be looking out for is there multiple success scenarios. In other
words, where I get a prime and non--prime. within each scenario. With multiplication it 's only
when we 're saying this scenario or this scenario that we add them together. That distinction can
sometimes be a little confusing. At first. We 're looking for the probability that we get a heads
and a six so it 's one half times one-sixth which is 1 over 12 and our answer is B good hopefully.
This is becoming a little clearer. Let 's keep going we 're going to start upping the difficulty a little
bit okay so probability for any given day is 1 6 to have rain..
The number of the chips do not matter no it totally does it matters that there's 12 of them and
then 11 of them. The threeness of it specifically matters no more than rolling a three on dice, so
as we 're doing these probabilities with multiple events we need to ask ourselves are we dealing
with replacement like rolling dice or flipping coins where the probability is constant. The
probabilities are n't going to change. Let 's try another one again. We 're rolling this dice twice.
We want the probability at least one of the rolls is greater than four so let 's think about the ways
this could happen. If either the first is more than four and the second is less than four. The
probability of each of these individually is going to be 1-6 times 1 6 for 1 over 36. The Other
solution. would be if we see this five success scenarios? We can ask how many total cases and
for that we would need to use combinatorics.. When We 've got multiple success scenarios and
you see something like at least or at most you want to ask yourself, which is easier successes
or failures.. The easier method is to do one minus failure because there are fewer failure
scenarios so that 's all the time we've got for right now. I 'm happy to answer any questions you
might have about the [UNK] or anything else..
