Question 1036244
It is the least common denominator for 15 and 25 minutes.  Multiplying them together will give A common denominator of 375 minutes.  It won't be longer than that.
The easiest way is to remember that the bus that departs every 25 minutes will leave 25,50,75,100... minutes. 
One only needs to look for where 15 divides evening into those numbers, and it is 75 minutes. 
One departs every x minutes and the other every 5x/3 minutes.  Five departures of 1 will equal 3 departures of the other: 75 minutes.