15. The distance in kilometers that the airplane has flown is expressed as a four-digit number divisible by 45, and the two middle digits are 39. Find this distance if it does not exceed 5000 km.
15. The desired number is divisible by 5, so it ends in the digit 5 or 0. It is also divisible by 9, so the sum of its digits is divisible by 9. Such numbers are 6390 and 1395. Since $6390>5000$, and $1395<5000$, the desired distance is 1395 km.