15. There is a four-story building (as shown in the right figure), with each of the 4 glass panes of each window painted either red or white, and each window represents a digit. Each floor has three windows, representing a three-digit number from left to right. The four floors represent the three-digit numbers: 791, 275, 362, 612. Question: Which three-digit number does the second floor represent?
15.【Solution】Among the 4 given numbers, 362 and 612 have the same last digit, and the symbols in the right windows of the second and fourth floors are also the same. Therefore, these two floors must represent 362 and 612, respectively. Since these two numbers also share the digit 6, by comparing the windows of the second and fourth floors, we can determine that the second floor represents 612.