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RD Sharma Solutions Class 7 Mathematics Solutions for Linear Equations In One Variable Exercise 8.4 in Chapter 8 - Linear Equations In One Variable
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A man says, “I am thinking of a number. When I divide it by 3 and then add 5, my answer is twice the number I thought of”. Find the number.
Answer:
Let the required number be x.
So, according to question:
⇒ x/3 + 5 = 2x
Transposing x/3 to RHS, we get
⇒ 5 = 2x – (x/3)
⇒ 5 = (6x – x)/3
⇒ 5 = (5x/3)
Multiplying both sides by 3 we get,
⇒ 5 × 3 = (5x/3) × 3
⇒ 15 = 5x
Dividing both sides by 5 we get
⇒ 15/5 = 5x/5
⇒ 3 = x
Thus the number thought of by the man is 3.
Video transcript
"Blahey kids how are you
i hope everyone's fine so
welcome to lido q a i'm vinit and i'm on
lido tutor
and we are here with another dose of
mathematics
for you okay so today what we are going
to do
today we are going to do this question
on the board
okay and solve it so this man
okay he says i am thinking of a number
when i divide it by 3 and then add 5
my answer is twice the number i thought
of
find the number so now this man is
challenging you
to find the number okay and he has given
you
hints what are the hints he says
when he divides it by 3 and then adds 5
to the answer
his answer is twice the number
okay so these are the numbers
of the number of the equation of the
question
okay yeah now
what we are going to do what is the
unknown here
x we have to find the number so
first of all okay right
solution okay
so let
the number
be x now what is it telling you
he is telling you when he divides it by
3
so x divided by 3
and then add 5 to the
question ok he gets
twice the number yes my answer
is means equal to twice the number
now we have x here
we have x on the right hand side so how
do we solve
equations when we have variables on both
the sides
simple we bring the variables on the
same side
and the numbers on the other side
so we can write this as 5 equal to
2x and since we are transposing
we perform the inverse operation so x by
3
right now these are unlike fractions
the denominator here is 1 the lcm of
these two are
3 the lcm here is 3
so what you do is you multiply the
numerator and the denominator
of 2x by 1 by 3
okay so when you multiply it what do you
get
5 is equal to 6 x
by 3 minus x by
3. so can we write okay now can we just
interchange to bring it in our standard
format
variables on the left side and once
the constant on the right side because
we are used to that method
so yeah let's do that because a plus
b is equal to 5 is the same as 5 is
equal to a plus b
yeah doesn't matter if we change the
size
so 6x minus x by 3 is equal to
5 okay now we transpose this 3
to the right hand side and perform the
inverse operation so this becomes 5x
is equal to 5 into 3 that is
15 okay again we transpose this
coefficient which is 5 and we take this
to the right hand side
and perform the inverse operation which
will be division
so x will be 15 by 5
which is equal to 3 so what is the
number that the person was thinking it
was
it is 3 okay now we can tell him the
answer
right now let's check the answer to be
sure right
so if he divides three by three
he gets one then he adds
five to 8 he gets 6
yes now 6
is again equal to 2 into
3 3 is the number he thought of twice
the number
so it satisfies the
conditions given by the man so yes
x is equal to 3 is the answer so i hope
this video helps you
and i hope you enjoyed the video so
don't forget to like
the video if you enjoyed the video and
if you have any doubts
please leave a comment below also don't
forget to subscribe to the channel
because we keep coming up with these
videos
and you have to be the first one to be
able to
watch this video and excel in your
mathematics
okay bye guys
nk"
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