Question 19
Vinh and Khang know only their own scores, natural numbers at most $20$. Vinh:“Our product is surely even.” Khang:“It is surely divisible by $20$.” Vinh:“Our scores differ and sum above $20$.” Khang:“I cannot determine yours, but our average exceeds mine.” Vinh:“I know yours, and our difference is divisible by $3$.” Khang:“Now I know yours.” Find Vinh’s score.