Question

Which one of the following ratios is proportional to 6:7?

• A. 3:17
• B. 9:15
• C. 18:21
• D. 6:12

Answer 1

The correct option is C 18:21

Two ratios are proportional when they are equal.
Writing all the options in simplest form,

(a) 3:17 is in its simplest form because there are no common factors between the antecedent and the consequent except 1.

(b) 9:15 = $\frac{3\times 3}{3\times 5}=\frac{3}{5}$ = 3:5

(c) 18:21 = $\frac{3\times 6}{3\times 7}=\frac{6}{7}$ = 6:7

(d) 6:12 = $\frac{1\times 6}{2\times 6}=\frac{1}{2}$ = 1:2

Clearly, 18:21 is proportional to 6:7.