21. Observant Yura noticed that if in a two-digit number expressing the distance in kilometers they traveled today, a zero is inserted between the tens and units digits, the resulting number is 9 times greater than the original. What distance did they travel?
21. Let the distance be expressed by the number $10 a+b$. If we insert a zero between the tens and units digits, we get $100 a+b$. According to the condition, we have $100 a+b=9(10 a+b)$, so $10 a=8 b$, which means $5 a=4 b, a=4, b=5$. Answer: 45 km.