Word problem regarding time

Question

Starting from the same spot, one boy completed a route in 9 minutes and an another boy completed it in 15 mins. When will both boys be at the same starting point at the same time again?

Answer 1

Notice that 9 = 3*3 and 15=3*5. Thus, to get both at the same point, you need to find all the multiples of 9 and 15. If you use 3*(3*5)=45, that solve the problem. In general any multiple of 45 also works.