All of the ratios in the table are equivalent to the ratio 3/5. Divide the numerator and denominator of 6/10 by 2, 24/40 by 8, 26/60 by 12, or 42/70 by 14 and you'll end up with 3/5.
Now just look through the answer choices and see which ratio is equal to 3/5. 18/20 simplifies to 9/10 which is not equal to 3/5. 30/20 simplifies to 3/2 which is not equal to 3/5. 18:30 simplifies to 3/5. 50/30 is equal to 5/3 which is not equal to 3/5.
Thus, C is the answer.
