Solution:
The whole numbers between 200 to 300 containing the digit 7 once and only once are, 207, 217, 227, 237, 247, 257, 267, 270, 271`, 272, 273, 274, 275, 276, 278, 279, 287, 297 = 18
Similarly between 300 to 400 = 18
Hence, total number of such numbers = 18 + 18 = 36
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