1. Muammo: Katta o‘g‘ilning yoshini toping. 2. Belgilaymiz: kichik o‘g‘ilning yoshi $x$. 3. Katta o‘g‘il kichik o‘g‘ildan 3 marta katta, ya’ni katta o‘g‘ilning yoshi $3x$. 4. Ota katta o‘g‘ildan 3 marta katta, ya’ni otaning yoshi $3 \times 3x = 9x$. 5. Ota kichik o‘g‘ildan 40 yosh katta, ya’ni $9x - x = 40$. 6. Tenglamani yechamiz: $8x = 40 \Rightarrow x = 5$. 7. Katta o‘g‘ilning yoshi $3x = 3 \times 5 = 15$. Javob: D) 15