Question

P can complete a round of circular track in 1 minute and 30 seconds while Q can complete a round in 45 seconds. If they start moving from the same point and move in opposite directions, then they meet after _ seconds.

• A. 45
• B. 60
• C. 30
• D. 90

Answer 1

Answer:

C. 30

Convert time into seconds:

• P takes 1 min 30 sec = 90 sec

• Q takes 45 sec

Their speeds (per round):

$P = ( \frac{1}{90} ) rounds/sec$

$Q = ( \frac{1}{45} ) rounds/sec$

Since they move in opposite directions, relative speed:
$\frac{1}{90} + \frac{1}{45} = \frac{1}{90} + \frac{2}{90} = \frac{3}{90} = \frac{1}{30}$

Time to meet:
$\frac{1}{\text{relative speed}} = \frac{1}{1/30} = 30 \text{ seconds}$

They meet after 30 seconds