If 18 is 15 percent of 30 percent of a certain number, what

Let the certain number be x.

=> (15/100)*(30/100)*x = 18;
450x=18*10000;
45x = 18000;
x=18000/45
=> x=400;

Ans is (E).

(15 * 30) / (100 * 100) = 0.045 , So if the number is 100, it will be 4.5. So the number has to be > 100. Hence option E)

If 18 is 15 percent of 30 percent of a certain number, what is the number?

(A) 9
(B) 36
(C) 40
(D) 81
(E) 400

18 is 15 percent of 30 percent of a certain number

Or, \(18 = (15%)(30%)x\)
Or, \(18 = (3/20)(3/10)x\)
Or, \(18 = (9/200)x\)
Or, \(x =(9*2*200)/9=400\)

Answer: (E)

Answer in 30 seconds by just glancing the answers.

As question states the 15% of 30% of x =18.

A)9 . Less than 18. Not an answer.
B)36 . 30 % is less than 18. Not an answer . Alternatively 1/3 (close to 30%) is 12 < 18. So not an answer
C)40 . 30 % is less than 18. Not an answer . Alternatively 1/3 (close to 30%) is 12 < 18. So not an answer
D)81 . 30 % is less than 18. Not an answer . Alternatively 1/3 (close to 30%) is 27 and 1/6 (close to 15%) 4 < 18. So not an answer
E)400 . E is answer no need to calculation.

Alternatively solution

Start putting the biggest number and get the result in first try 15% of 30% of 400 =18.

Cheers..

Hi All,

The math involved in this question can be 'set up' (and dealt with) in a variety of ways.

Here, if you're going to take a calculation-based approach, converting the percents to fractions would likely help you speed up (and avoid the last bit of math).

We're told that 18 is 15 percent of 30 percent of a certain number. We're asked for that number.

18 = (.15)(.3)(X)

18 = (3/20)(3/10)(X)

18 = (9/200)(X)

Since 9/200 is a relatively small fraction (it's far less than 10%), we know that 18 is LESS than 10% of X. Based on the given answer choices, there's only one number that is LARGE enough to fit that description....

Final Answer:

GMAT assassins aren't born, they're made,
Rich

Solve as question 15% of a number is 18 so

Here we can say that 10% = 12

And 100% = 120

And by the question you can see that this is 30% of the no that we are looking for

So the no is 400 as 120 is 30 % of 400


Hope this helps


Sent from my iPhone using GMAT Club Forum mobile app

18 = 0.15 * 0.30 * X

Answer = E

Can someone provide me a tip as to how to calculate 18 / 0.045 without a calculator? Bunuel

Hint: notice that 18 = 2*9 = 4*4.5

Answer is Clearly E

let the number is N
N x 0.30 x 0.15 = 18
N = 400
which is E

We need to find a number such that
15 percent of 30 percent of a that number is 18

Let the number be x

0.15*0.3*x = 18
0.045x = 18
x = \(\frac{18}{0.045} = \frac{180 * 100}{45}\) = 400(Option E)

In the Stem it is asked about 18 = 15 % of 30 % of a number, 15% of a number is 18 means the number should be greater than 100, only answer E having number greater than 100, so answer should be E

but you must calculate the answer , if in case the answer choices are above 100 then your method will not work, so my suggestion is although here it is applicable but when there are comparable choices do by actual calculation.

Solution:

Letting x = the certain number, we can create the equation:

18 = 0.15 * 0.3 * x

18 = 0.045x

18000 = 45x

x = 18000/45 = 400

Answer: E

\(18 = \frac{15}{100}*\frac{30}{100}*x\)

So, \(x = \frac{18*100*100}{15*30}\)

Or, \(x = 400\), Answer must be (E)

Agree but one can apply many filters again to make a good guess. Depends on the options. One such filter is making sure that answer ends with zero.