A radio station gives away $15 to every 15th caller, $25 to every 25th caller and free concert tickets to every 100th caller. When will the station first give away all three prizes to one caller?

I suspect you are studying prime factors, right? You need to find the least common multiple of the numbers 15, 25 and 100. Do you know how to do that? It involves breaking each of these numbers into its prime number constituents, then combining the primes so that each is represented the maximum number of times it occurs in any one of the numbers. Try it and show us what you get.