Two friends meet at a metro station. One friend says, “The product of my three sons’ ages is 72.” The other friend replies, “That’s not enough information to determine their ages.” The first friend then adds, “The sum of my three sons’ ages is equal to the number of the metro platform we are standing on.” The second friend thinks for a moment and says, “I still cannot determine their ages.” Finally, the first friend says, “My eldest son has started learning piano.” Immediately, the second friend says, “Now I know the ages of your three sons.”