Risa and David started off from the same place and drove at uniform speeds in the same direction round a 25 km circular racing track. Risa completed each round in 60 minutes. David took 100 minutes to complete each round.
- How far would David be behind Risa after 1 hour?
- How long after they started would it take Risa to meet David for the first time? Express your answer as a whole or mixed number in h.
(a)
60 min = 1 h
1 min =
160 h
60 min =
6060 h = 1 h
100 min =
10060 h = 1
23 h
Risa's speed
= 25 ÷ 1
= 25 x
11= 25 km/h
David's speed
= 25 ÷ 1
23= 25 x
35= 15 km/h
Distance between Risa and David after 1 hour
= 25 - 15
= 10 km
(b)
LCM of 60 and 100 = 300
300 min =
30060 h = 5 h
Time needed for Risa to meet David for the first time = 5 h
Answer(s): (a) 10 km; (b) 5 h