Archie, Dylan and Seth started walking at the same time from the same starting point round a circular track. Seth walked in the clockwise direction. Archie and Dylan walked in an anti-clockwise direction. Seth took 12 min to complete each round. He met Archie after every 2 minutes. He met Dylan after every 7 minutes. The three friends kept to their respective speeds throughout their walk.

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

(a)
1 h = 60 min

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

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

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

Seth's distance : Archie's distance
2 : 12 - 2
2 : 10

Seth's distance : Dylan's distance
7 : 12 - 7
7 : 5

Seth	Archie	Dylan
2x7	10x7
7x2		5x2
14	70	10

The distance of Seth is the repeated identity. 
LCM of 2 and 7 = 14

In 1 h
Seth's rounds : Archie's rounds : Dylan's rounds
14 : 70 : 10

Distance that Dylan completed in 1 h = 2.4 km

10 rounds = 2.4 km = 2400 m

1 round = 2400 ÷ 10 = 240 m

Circumference of the circular track = 240 m

Answer(s): (a) 5; (b) 240 m