Question

If a : b :: c : d then which of the following is correct?

• A. a:b :: d:c
• B. b:a :: c:d
• C. b:a :: d:c
• D. None of these

Answer 1

The correct option is C

b:a :: d:c

If a:b::c:d then $\frac{a}{b}=\frac{c}{d}$
which can be written as $\frac{b}{a}=\frac{d}{c}$
i.e., b : a :: d : c
Therefore a : b :: c : d can be written as b : a :: d : c
This is called the invertendo property of proportions.