When a runner was crossing the 12 km mark, she was informed that she had completed only 80% of the race. How many kilometres was the runner supposed to run in this event?

The question states that 12 km is 80% of the total race distance. I need to find the total distance, which is 100%. This is a very standard CSAT question type. A similar problem appeared in 2014 involving bus fare, where a given amount of money was 80% of the total [3, 57]. The best way to solve this is not with equations, but with fractions. 80% is the same as 80/100, which simplifies to 4/5. This is a mental math shortcut that saves a lot of time [28]. So, 4/5th of the race is 12 km. If 4 parts are equal to 12 km, then 1 part must be 12 divided by 4, which is 3 km. The total race is 5 parts. Therefore, the total distance is 5 parts multiplied by 3 km per part, which equals 15 km. Looking at the options, (B) is 15. A quick check: 80% of 15 is (4/5) * 15 = 12, which matches the question. So the answer is correct.