Question 980384
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Car X crosses the start line every 15 minutes:
10km/40km/hr=1/4 hr=15 minutes
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Car Y crosses the start line every 20 minutes:
(assuming speed 30km/hr instead of 30mm/hr)
10km/30km/hr=1/3 hr=20 min
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The prime factors of 15 are 3 and 5 (3 x 5=15)
The prime factors of 20 are 2 and 5 (2 x 2 x 5=20)
Take the shared prime factor (5) and multiply it times all other prime factors from both numbers: 5 x 3 x 2 x 2=60
The least common multiple is 60 minutes.
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Since the least common multiple of 15 and 20 will be 60,
both will cross the line together in one hour (60 minutes).