3/4 cm:12km=2cm:N km
First,you divide the extremes of the first ratio to the extremes of the second ratio.
Ex.12 divided by 3
Then you multiply the denominator (if fraction) of the extremes of the extremes of the second ratio to the means of the first ratio.
Ex.12divided by 3 =4
4 times 2 = 8
Finally,multiply the answer to the denominator of the means of the first ratio.
Ex.8 times 4 =32km
The answer on the question is 32 km.