The GMAT DS question given below is Number Properties question and the concept covered is test of divisibility of numbers and remainders of the division.
This data sufficiency problem consists of a question and two statements, labeled (1) and (2), in which certain data are given. You have to decide whether the data given in the statements are sufficient for answering the question. Using the data given in the statements, plus your knowledge of mathematics and everyday facts (such as the number of days in a leap year or the meaning of the word counterclockwise), you must indicate whether -
All numbers used are real numbers.
A figure accompanying a data sufficiency question will conform to the information given in the question but will not necessarily conform to the additional information given in statements (1) and (2)
Lines shown as straight can be assumed to be straight and lines that appear jagged can also be assumed to be straight
You may assume that the positions of points, angles, regions, etc. exist in the order shown and that angle measures are greater than zero.
All figures lie in a plane unless otherwise indicated.
In data sufficiency problems that ask for the value of a quantity, the data given in the statement are sufficient only when it is possible to determine exactly one numerical value for the quantity.
Question 11: Is the positive integer X divisible by 21?
What kind of an answer will the question fetch?
The question is an "Is" question. Answer to an "is" question is either YES or NO.
When is the data sufficient?
The data is sufficient if we are able to get a DEFINITE YES or a DEFINITE NO from the information given in the statements.
If we get a MAYBE as an answer, the data is NOT sufficient
Do we have any more information about 'X' from the question stem?
The question stem states that 'X' is a positive integer.
What kind of numbers will be divisible by 21?
A number is divisible by 21 if it is divisible by 3 and 7.
The number is, therefore, of the form 14k + 4.
It will leave a remainder of 4 when divided by 7. (14k is divisible by 7. When 4 is divided by 7, the remainder is 4.)
This number is definitely not divisible by 7.
To be divisible by 21, the number must be divisible by both 3 and 7. This number is not divisible by 7. Hence, X is not divisible by 21.
We have a DEFINITE NO as the answer to the question using statement 1.
Statement 1 ALONE is sufficient.
Eliminate choices B, C, and E. Choices narrow down to A or D.
The number X is of the form 15m + 5
Therefore, the number will leave a remainder of 2 when divided by 3.
Hence, it is not divisible by 3.
To be divisible by 21, the number must be divisible by both 3 and 7. This number is not divisible by 3. Hence, X is not divisible by 21.
We have a DEFINITE NO as the answer to the question using statement 2 as well.
Statement 2 ALONE is also sufficient.
Eliminate choice A.
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