Ivan, Reggie and David started walking at the same time from the same starting point round a circular track. David walked in the clockwise direction. Ivan and Reggie walked in an anti-clockwise direction. David took 12 min to complete each round. He met Ivan after every 2 minutes. He met Reggie after every 6 minutes. The three friends kept to their respective speeds throughout their walk.

1. How many rounds did David complete in 1 h?
2. When David and Ivan met again at the starting point after 1 h, Reggie had completed 2.4 km. Find the circumference of the circular track in metres.

(a)
1 h = 60 min

Number of rounds that David completed in 1 h
= 60 ÷ 12
= 5

(b)
David took 12 min to complete 1 round
1 min = ¹₁₂ of circular track = 1 u
12 min = ¹²₁₂ of circular track = 12 u

David's distance is the repeated identity.
Make David's distance the same.

David's distance : Ivan's distance
2 : 12 - 2
2 : 10

David's distance : Reggie's distance
6 : 12 - 6
6 : 6

David	Ivan	Reggie
2x3	10x3
6x1		6x1
6	30	6

The distance of David is the repeated identity. 
LCM of 2 and 6 = 6

In 1 h
David's rounds : Ivan's rounds : Reggie's rounds
6 : 30 : 6

Distance that Reggie completed in 1 h = 2.4 km

6 rounds = 2.4 km = 2400 m

1 round = 2400 ÷ 6 = 400 m

Circumference of the circular track = 400 m

Answer(s): (a) 5; (b) 400 m