But the next problem stopped him cold. Problem 790: A different father is four times as old as his son. In 18 years, he will be only twice as old. But the sum of their current ages is a prime number. Find their ages.
He had learned something the culegere never said out loud: sometimes the right answer is that there is no answer—and explaining why is the real solution. culegere matematica clasa a 9 a
One rainy Thursday, he flipped to a random page. Problem 789: A father is three times as old as his son. In 12 years, he will be twice as old. Find their ages. But the next problem stopped him cold
He checked twice. No mistake. He checked the answer key at the back—it only said “Impossible. Explain why.” But the sum of their current ages is a prime number
Andrei hated the culegere . Its thick, blue cover—creased at the spine, coffee-stained on the back—sat on his desk like a small, mute tyrant. His father had bought it in September with the best intentions: “Three problems every night, and you’ll be top of the class.”
“The equations force the son to be 9 and the father 36, with sum 45. Since 45 is composite (3 × 15, 5 × 9), the condition ‘sum is prime’ cannot be met. Therefore, no such ages exist in whole numbers.”