Okay, so now to number 20. There is a single train track between points X and Y as we see in this image over here, X to Y. Here's our train track. A train company wants one train to leave from X and one train to leave from Y at the same time daily. Moving with constant speed, it takes 180 minutes for a train to make a trip from X to Y and 60 minutes from Y to X. They want to build a double track as shown here to avoid a crash. Where should the double track be? Okay, so let's assume that each segment separated by dots is 10 units just for the sake of simplicity. So X to Y, we have 10, 20, 30, 40, 50, 60, so distance X, Y is 60 units. Now what do we know? We know that the train that travels from Y travels three times faster than train X. So when are these trains going to meet? Well, train Y will meet when train X, when Y has traveled 45 units and X has traveled 15 units, right? Train Y travels three times faster. So the sum of train Y and train X's distance will be 60 when Y is three-fourths of 60, X is one-fourth of 60. Okay, so where is one-fourth, or sorry, where is 15 and 45 units on this railroad? Well, this is 10, so 15 is somewhere here, and the 45 from Y is 10, 20, 30, 40, also right here. So they'll intersect right about here, and that's why B makes the most sense to put the double track there. That's where they'll intersect. The answer is B.

train collision prevention

double track installation

train speed comparison

safety measures

recommended track location