Maths : Basic Numeracy

Q 224 / 370

UPSC CSE Prelims 2019

If the numerator and denominator of a proper fraction are increased by the same positive quantity which is greater than zero, the resulting fraction is

EXPLANATION

Correct Option (2)

Let the original proper fraction be represented as ab, where a and b are positive integers such that a < b. This condition signifies that the fraction's value is less than 1.

Let the positive quantity by which both the numerator and denominator are increased be k, where k > 0. The resulting fraction is a+kb+k.

To compare the original fraction ab with the new fraction a+kb+k, we can examine their difference:

a+kb+k−ab=b(a+k)−a(b+k)b(b+k)

=ab+bk−ab−akb(b+k)

=bk−akb(b+k)

=k(b−a)b(b+k)

Given that ab is a proper fraction, a < b, which implies that (b−a) is a positive quantity. Additionally, k > 0, b > 0, and (b+k) > 0. Therefore, the entire expression k(b−a)b(b+k) is always positive.

Since the difference \frac{a+k}{b+k} - \frac{a}{b} > 0, it follows that \frac{a+k}{b+k} > \frac{a}{b}. Thus, the resulting fraction is always greater than the original proper fraction.

Incorrect Options:

  • Option 1 (always less than the original fraction): This statement is incorrect. As demonstrated by the algebraic analysis, adding the same positive quantity to both the numerator and denominator of a proper fraction consistently increases its value, moving it closer to 1.

  • Option 3 (always equal to the original fraction): This statement is incorrect. For the fractions to be equal, the difference k(b−a)b(b+k) would need to be zero. This is not possible because k > 0 and (b - a) > 0 for a proper fraction.

  • Option 4 (such that nothing can be claimed definitely): This statement is incorrect. A definite conclusion can be drawn based on the mathematical properties of proper fractions and the specified operation. The resulting fraction is consistently greater than the original.

Explanation figure for the question: If the numerator and denominator of a proper fraction are increased by the same positive…