Relative speed = 5+10 = 15 kms/hr

Time taken at which 2 boats collide is after = 20/15 = 4/3 = 1 hr 20 min (or) 80 min

1 min before, distance covered is
20 - 20*60/79 = 20 - (~ 15 kms) = 5 kms or 1/4th of the total distance.

papgust wrote:Relative speed = 5+10 = 15 kms/hr

Time taken at which 2 boats collide is after = 20/15 = 4/3 = 1 hr 20 min (or) 79 min

1 min before, distance covered is
20 - 20*60/79 = 20 - (~ 15 kms) = 5 kms or 1/4th of the total distance.

The answer is 1/4 of a km, not 1/4 of the total distance.

We can answer this question very quickly if we understand relative rates.

Here's the rule to apply:

When two objects are moving in opposite directions, ADD the rates to get the total rate.

As Papgust noted, the total rate is 15kph.

Now, when the two objects collide in a ball of fire, d=0. To solve the question we simply need to figure out how far they travelled in that final minute.

t = 1/60
r = 15
d = r*t = 1/60 * 15 = 15/60 = 1/4 of a km... choose C.