In DS questions like these, it becomes evident from the looks of one of the statements that it will not be sufficient when taken alone. In this question, for example, Statement II is insufficient when taken alone since it gives us a range for a whereas we are required to find a definite value for a.
This gives us the opportunity to eliminate some options quickly and you should always be on the lookout for such chances. We can eliminate options B and D. The possible answers are A, C or E.
Let us now analyse statement I. Statement I talks about two numbers: a and (a+1). These are two consecutive numbers. Any two consecutive numbers will always be co-prime and will not have any other factor in common except 1.
Also, of two consecutive numbers, one will always be odd and the other even. This is a lot of data. Statement I also says that a and (a+1) have exactly 6 factors each. This means that both a and (a+1) are composite numbers.
The general form of a composite number is N = \(a^p * b^q*c^r*\)…… For such a number, the number of factors is given by the expression (p+1) (q+1) (r+1)…..
Applying this concept to our numbers a and (a+1), we can say that (p+1) (q+1)(r+1)………. = 6. This can be done in 2 ways:
Case 1:
If a = \(x^p\) where p = 5, then a will have 6 factors. Depending on the value of x, a can be 1 or 32 or 243 and so on. (a+1) will be 2 or 33 or 244 and so on. The numbers 2 and 33 do not have 6 factors, whereas 244 has 6 factors. So, do we decide that 243 and 244 are our numbers?? Certainly not because depending on the value of x, there can be other values of a and (a+1) that can have 6 factors each.
Case 2:
If a = \(x^p * y^q\), then (p+1) * (q+1) = 6, which can be satisfied by taking p = 2, q = 1 or vice-versa. In this case also, since we don not have an upper limit on a, we can have multiple cases satisfying our condition.
We can safely conclude that statement I alone will be insufficient. Answer option A can be eliminated, the possible answer options at this stage are C or E.
Combining statement I and II, it’s a great thing that we have a limit on 76 because we will now have to deal with a specific set of numbers.
As per case 1, a can be 1 or 32 only since 81 or 243 are greater than 76.
If a=1, (a+1) = 2. None of these numbers have 6 factors.
If a = 32, (a+1) = 33. 33 does not have 6 factors.
We conclude that a cannot be of the form \(x^p\).
Let us look at numbers which can be expressed in the form of \(x^p * y^q\). We can work this out again on a case to case basis.
If x =2, p=2 and y=11, q=1, then a = \(2^2 * 11^1\) = 44 and (a+1) = 45. In this case, a and (a+1) have 6 factors each.
If x = 5, p=2 and y=3, q=1, then a = \(5^2 * 3^1\) = 75, and (a+1) = 76. In this case again, a and (a+1) have 6 factors each.
Clearly, we don’t have a unique value of a. The combination of statements is insufficient, answer option C can be eliminated.
The correct answer option is E.
The number of factors of a composite number does not necessarily depend on the magnitude of the number. As we saw, a smaller number can have the same number of factors as a larger number. This is because the number of factors of a composite number depends on the exponents of the prime factors and not so much on the prime factors themselves.
Hope that helps!
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Crackverbal Prep Team
www.crackverbal.com