216 –1 is divisible by

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∵ 2¹⁶ – 1 = (2⁸)²–1
= (2⁸+1) (2⁸–1)
= (256 + 1) (256 – 1)
2¹⁶ – 1 = 257 × 255 which is exactly divisible by 17.