Leah here from leah4sci.com/MCAT and in this
video I will show you how to quickly and easily
Multiply and Divide numbers by a factor of
10 without a calculator.
You can find my entire series on doing MCAT
Math Without A Calculator by visiting my website
http://leah4sci.com/MCATMATH.
Multiplying and Dividing by factors of 10
will come up again and again in your MCAT
practice including questions related to everything,
from Unit Conversions to General Math. Multiplying
and Dividing by 10 is a fun way to do Math
in your head and 10 comes up in a lot of questions
from converting something like grams to kilograms,
solving gravitational acceleration questions
and so much more. Or say your number does
not have the factor of 10, round it to the
nearest 10 and then apply the tricks. Why?
Because it's easier and faster than doing
it the long way.
Now before I show you how to find the solution
of 3.47 x 1000 in under 5 seconds, let's
do a fun exercise. Say I ask you to perform
the calculation of 2.3 x 10 in your head.
Some of you can do it, others will look at
me like I'm crazy! Now maybe I am, but that's
not the topic for today. If you got it, great!
If not, let's try this quick exercise. If
I ask you 1 x 10, I hope you know that the
answer is 10. If you didn't, you need to
download my MCAT Math guide by visiting my
website http://leah4sci.com/MCATMATH. Now
if I ask you to do 2 x 10, the answer is 20.
If I ask you to do 3 x10, the answer is 30.
These are simple equations, but I want you
to focus not on the answer but think about
what you did.
You have a number and you multiplied it by
10 so all you did was add a zero. We took
the number 3, we added the 0, that's how
we got 30. Now if I ask you to do 20 x 10,
once again, all I do is add a zero (0), that
gives me Two Hundred Thirty (230). In other
words, to multiply any number by 10, all you
have to do is add a zero (0).
So what if I ask you to do 2.3 x 10 or we
can't really add a zero because we have
a decimal. So let's revisit the initial
trick; when we added a zero, all we really
did was move that imaginary decimal at the
end of the number over one space to the right.
Starting with 1, we move it over that gave
us 10, 2 gave is 20, 3 gave us 30 and 23 gave
us 230. So if the decimal is not invisible,
the same trick applies. Take the decimal,
move it over one space to the right and 2.3
now becomes 23. Let's crank it up a notch.
What if I ask you to solve 37 x 100? We�re
not dealing with 10 now we are dealing with
a hundred (100) which is 10 x 10. Anytime
you have the number 1 followed by zeros (0),
that number is simply 1 x 10 again and again
until it equals the number of zeros that you
have. Looking at the number 100 we have two
zeros (0), that means we're multiplying
by two factors of 10 so we'll take that
imaginary decimal at the end of number and
move it over 2 spaces, one for each zero giving
me an answer of 3700.
Now let's revisit our initial question.
This may show up as part of a larger question
asking you to convert 3.47 kilograms into
grams. So we take our initial value of 3.47kg.
I'm writing this out to show you but I want
you to set this up mentally. Knowing that
there's a thousand (1000) grams every kilogram
we set it up so kilograms can cancel and now
our equation is simply 3.47 x 1000. A thousand
has 3 zeros which means we move the decimals
3 spaces to the right, one, two, three,
giving as an answer of 3470. This example
when done in your head should take you under
5 seconds!
Now let's say we want to divide a number
by a factor of 10. If multiplying by 10 means
you move the decimal to the right, then dividing
by 10 means you move the decimal to the left.
Let's start simple: 30 divided by 10, we
take the decimal, move it one space to the
left that gives us 3. Given 470 divided by
100, one hundred (100) has two zeros so we
take the decimal move it 2 spaces to the left,
that gives us 4.7.
Now what if you're starting with the decimal?
Say you have 0.53 and you want to divide it
by 1000. For example you have 0.53 grams and
you want to turn this into kilograms. We have
three zeros dividing by 1 and three zeros
means move the decimal three spaces to the
left, one, two, three. Be very careful to
count your zeros here, the answer is 0.00053.
you'll probably see this in Scientific notation
and that's something I will cover in the
future video.
Now what if the example given isn't as clean,
meaning you don't see the 10 as obvious?
So let's make up a number; Say we have 3.29
and we want to multiply this by 9.73. When
9.73 is very close to 10 and on the MCAT,
close enough is good enough. 9.73 is 10 that
means we move the decimal one space to the
right giving me an answer of 32.9.
Remember the choices on the MCAT won't be
very very close to each other because they�re
testing your ability to do the right kind
of Math and give you wrong answers that are
simply all over the place allowing you to
rule out anything that is way too far off
from the number that you got.
Be sure to join me in the next video where
I take this concept to the next level, showing
you how to quickly multiply and divide numbers
using the factor of 10 trick when you can
even round your number to the nearest 10.
Are you stuck on a specific MCAT topic? I
offer Private Online Tutoring where I focus
on your needs to strengthen your individual
weaknesses. Tutoring details can be found
using the link below or by visiting my website
leah4sci.com/MCATTutor.
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