Carl, Billy and David started walking at the same time from the same starting point round a circular track. David walked in the clockwise direction. Carl and Billy walked in an anti-clockwise direction. David took 5 min to complete each round. He met Carl after every 3 minutes. He met Billy after every 4 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 Carl met again at the starting point after 1 h, Billy had completed 2.7 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 ÷ 5
= 12

(b)
David took 5 min to complete 1 round
1 min = ¹₅ of circular track = 1 u
5 min = ⁵₅ of circular track = 5 u

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

David's distance : Carl's distance
3 : 5 - 3
3 : 2

David's distance : Billy's distance
4 : 5 - 4
4 : 1

David	Carl	Billy
3x4	2x4
4x3		1x3
12	8	3

The distance of David is the repeated identity. 
LCM of 3 and 4 = 12

In 1 h
David's rounds : Carl's rounds : Billy's rounds
12 : 8 : 3

Distance that Billy completed in 1 h = 2.7 km

3 rounds = 2.7 km = 2700 m

1 round = 2700 ÷ 3 = 900 m

Circumference of the circular track = 900 m

Answer(s): (a) 12; (b) 900 m