# Moderate Time, Speed & Distance Solved QuestionAptitude Discussion

 Q. Two boats, traveling at 5 and 10 kms per hour, head directly towards each other. They begin at a distance of 20 kms from each other. How far apart are they (in kms) one minute before they collide?
 ✖ A. $\dfrac{1}{12}$ ✖ B. $\dfrac{1}{6}$ ✔ C. $\dfrac{1}{4}$ ✖ D. $\dfrac{1}{3}$

Solution:
Option(C) is correct

The boats will be colliding after a time which is given by:

$=\dfrac{20}{5+10}$

$=\dfrac{4}{3}$ hr

$=80$ min

After this time of 80 minutes, boat (1) has covered:

$=80\times \dfrac{5}{60}$

$=\dfrac{20}{3}$km

whereas boat (2) has covered:

$=80\times \dfrac{10}{60}$

$=\dfrac{40}{3}$ km

After 79 minutes, distance covered by the first boat ($D_1$):

$=\left(\dfrac{20}{3}-\dfrac{5}{60}\right)$

After 79 minutes, distance covered by the second boat ($D_2$):

$=\left(\dfrac{40}{3}-\dfrac{10}{60}\right)$

So the separation between the two boats:

$= 20-(D_1-D_2)$

$=\dfrac{1}{4}$km

Alternative Method:
Relative speed of two boats:

$=5+10=15$ km/hr

i.e. in 60 min they cover (together) = 15 km

In 1 min they will cover (together) = $=\dfrac{15}{60}=\dfrac{1}{4}$ km

## (1) Comment(s)

Rohit Sharma
()

alternate solution:

relative speed of boats: 5 kmph +10 kmph = 15 kmph

we have to find the distance between two boats 1 min before colliding:

distance before colliding = distance they have to cover to collide

=> distance 1 min before colliding = distance to be covered in 1 min

so basically we have to find the distance to be covered in 1 min, i.e,

in 1 hr => 15km

in 60min => 15km

in 1 min => (15/60)km

=> (1/4)km