Answer:
7/14 and 20/40 are proportional.
Explanation:
We're looking for the pairs of ratios where the two ratios are equal.
6/21 and 2/42 are not equal, although 6/21 and 12/42 would be equal.
25/100 and 50/150 are not equal.
18/24 and 6/12 are not equal. Notice that dividing 18 and 24 by 6 results in 3/8, which does not equal 6/12 or 3/6 or 1/2.
In the last case, notice that 7/14 = 1/2 and 20/40 = 1/2. This pair of fractions is proportional. Note that 20/7 times 7 is 20 (the numerator of the 20/40), and that 20/7 times 14 is 40. The constant of proportionality is thus 20/7.