Question

two bike riders ride around in a circular path. The first rider completes one round in 15 minutes and the second rider completes it in 18 minutes. if they both start together and ride the same route, after how many minutes will they meet again at the starting place​

Answer 1

Rider 1 does one round in 15 min, and will complete another in each consecutive multiple of 15 min

Rider 2 does one round in 18 min, and will complete another in each consecutive multiple of 18 min

Assuming that they start together, they will complete another round together in a time that is both multiples of 15min and 18 min.

Then we need to find the smallest common multiple between 15 and 18.

To smallest common multiple between two numbers, a and b, is equal to:

a*b/(greatest common factor between a and b).

Now, the greatest common factor between 15 and 18 can be found if we write those numbers as a product of prime numbers, such as:

15 = 3*5

18 = 2*3*3

The greatest common factor is 3.

Then the smallest common multiple will be:

(15*18)/3 = 90

This means that after 90 mins, they will meet again at the starting place.