Data Sufficiency Video Lecture - GMAT

Let's continue with the GMAT
problems. We're on problem 16
on page 279.
Problem 16.
Is x greater than 1.8?
That's all they're asking us.
And statement one says,
x is greater than 1.7.
Well that doesn't help us
because I don't know, x could
be 1.71, in which case this
wouldn't be true.
Or x could be a million, in
which case this is true.
So this isn't really
that helpful.
We immediately see that.
Statement two says that
x is greater than 1.9.
Well, if you're greater than
1.9, you're definitely
greater than 1.8.
So this is all we need,
statement number two.
So b.
Problem 17.
Problem 17.
If n is an integer,
is n plus 1 odd?
Well if n plus 1 is odd, that's
the same thing as
asking, is n even?
And I haven't looked at the
statements yet, but that's
just something your brain might
automatically connect.
And let's see.
What do they tell us?
Statement one tells us that
n plus 2 is even.
Well if n plus 2 is even, then
n is definitely even.
If you don't believe me, think
about the number 4.
4 is even, and 4 plus 2 is
6, which is still even.
Right?
Because if you add 1, you're
going to go from even to odd.
You add 2, you go back
to even, right?
So this is all we need.
If n plus 2 is even, then we
know that n is even, and n
plus 1 is odd.
And you could try it out
with numbers if you
don't believe me.
Two.
What is statement two?
n minus 1 is odd.
Well this is the same thing.
n minus 1 is odd.
Once again this tells us that
n has to be an even number.
If you subtract 1 from it and
you get an odd number, then it
has to be even.
If you say that n is
4, 4 minus 1 is 3.
And try it for any
even number.
I don't want to over-explain
with a fairly simple problem.
So anyway, the answer is either
of them alone are
sufficient, D.
And try it out with numbers
if you don't believe me.
Problem 18.
Is x between 1 and 2?
Is essentially what
they're saying.
It can't be 1 or 2, because
these aren't equal signs,
they're less than signs.
OK.
So that's what they're asking.
Statement one tells us that x is
essentially greater than 0.
They wrote it a little
different, they said 0
is less than x.
So x is greater than 0.
That doesn't help us. x
could still be 0.1 It
could be still be 1/2.
So that doesn't tell us where
it is in this range.
It could also be 100, in which
case it's out of this range.
Statement two tells us that
x is less than 3.
So this alone doesn't help us.
x could still be 2.1.
Or x could still be
minus a million.
This doesn't help us.
And even if we took them
together, that would just tell
us that 0 is less than x,
which is less than 3.
Which is a superset
of this, right?
If we know that x is in this
range, it doesn't tell us that
x is definitely in this range.
For example, if we know this
is true, x could still be
equal to 1/2.
But 1/2 isn't in this range.
x could be equal to 1.5,
which is in the range.
Or x could be equal to 2.5,
which isn't in the range.
So both of these, even taken
together, are not sufficient.
So the answer is e.
Problem 19.
These are going fast. Maybe too
fast. Let me know if I'm
going too fast. OK.
Water is pumped into a partially
filled tank at a
constant rate through
an inlet pipe.
At the same time, water is
pumped out of the tank at a
constant rate through
an outlet pipe.
OK.
So essentially we have a tank.
There's a pipe here.
So water is going in at, x-- we
could say, I don't know--
meters cubed per second or--
units shouldn't matter.
And it's getting pumped out at
y meters cubed per second.
At what rate in gallons per
minute-- OK, so these aren't
meters cubed per second, these
are gallons per minute-- at
what rate in gallons per minute
is the amount of water
in the tank increasing?
So in order for the water to be
increasing, more has to be
coming in than going out.
And the rate of it is going
to be the difference.
So essentially they just want to
know what x minus y gallons
per minute are.
If x is less than y, and we get
a negative number here,
then actually we have more
coming out than going in.
And so the water isn't
increasing.
So anyway, they tell us that,
one, the amount of water
initially in the tank
is 200 gallons.
That's useless.
Why is that useless?
Because it just tells us how
much water is in the tank.
It doesn't tell us anything
about the
rates going in or out.
So that's useless.
Two.
Water is pumped into the
tank at a rate of
10 gallons per minute.
So x is equal to 10 gallons
per minute.
That's the rate you're
pumping into it.
And out of the tank
at a rate of 10
gallons every 2 1/2 minutes.
So y is equal to 10 gallons
per 2.5 minutes.
Which is equal to what?
That's 4 gallons per minute.
So that second statement, that
second part of statement two
is a little bit shady,
but you get a y.
It's 4 gallons per minute.
So you actually don't
have to figure it.
But if you wanted to, you could
say that the rate at
which water is increasing is--
the rate at which it's coming
in at 10, minus the rate
it's going out.
So 6 gallons per minute
is actually the
answer to the question.
But all you have to know is that
you just needed statement
two, which gives us this
information to figure it out.
And statement one was useless.
So that's b, statement two
alone is sufficient.
Let's do another one.
OK.
So we have problem number 20.
Is x negative?
OK.
The first statement, they
tell us that 9x is
greater than 10x.
So this is an interesting
situation.
So let's try to solve
this equation.
Let's subtract 10x from both
sides of this equation.
And if you're adding or
subtracting on both sides of
an equation or inequality, the
inequality stays the same.
So let's subtract 10x from
both sides of this.
So you get 9x minus 10x is
greater than 10 minus 10x.
And then you get minus
x is greater than 0.
Or you can multiply both
sides by negative 1.
And when you multiply or divide
by a negative number
with an inequality, you
switch the inequality.
x is less than 0, so we know
that x is definitely negative.
The other way you could have
done it, you could have
subtracted 9x from both sides,
and you would have gotten 0 is
greater than x.
Which is a faster
way of doing it.
But either way, statement one
lets us know that x is
definitely negative.
Let's see what statement
two does for us.
Statement two.
x plus 3 is positive.
Well I mean, x could be 100.
If x is 100, then 103 is
definitely positive.
Or x could be negative
1, right?
Because negative 1 plus
3 is positive 2.
So this doesn't tell us
any information about
whether x is negative.
So this is useless.
So the answer is a.
Statement one alone
is sufficient.
Statement two alone
is useless.
Let's see how much time I have.
I'm doing well on time.
I'm on problem 21.
Does 2m minus 3n equal 0?
And let's just think
about this.
That's the same thing as asking,
does 2m equal 3n?
If you just add 3n into
both sides, these
are equivalent questions.
If you can answer one, you
can answer the other.
So statement one says,
m does not equal 0.
Well that seems fairly
useless to me.
I mean, m does not equal 0, so
m is still-- I could pick a
random m, not equal to 0, and
depending on what n is, this
may or may not be true.
So far that seems kind of
useless for me, but maybe it's
useful in conjunction
with two.
The next thing they say,
6m is equal to 9n.
This one's interesting.
So what does this tell us?
So 6m is equal to 9n.
So let's divide both
sides by 3.
We get 2m is equal to 3n.
And then we can subtract
3n from both sides.
And you get 2m minus
3n is equal to 0.
So statement two-- I mean, you
essentially just do a little
bit of algebra, and you get what
we're trying to prove.
So statement two is good.
Statement one, do we
need it at all?
Well no, because-- I mean, that
is one solution, that if
m and n are both equal to 0,
that this thing is true.
But it doesn't really
do as much in the
way of anything else.
And we don't need it, to
come up with this.
So the answer is b.
Statement two alone
is sufficient.
I'm already past 10 minutes.
So that's it.
I'll do 22 in the next video.
See you soon.