Pull out a common factor of 10⁷:
10⁹ + 10⁸ + 10⁷ = 10⁷ (10² + 10¹ + 10⁰)
… = 10⁷ × 111
Factorize 10 as 2 × 5, then distribute the power of 7:
… = 2⁷ × 5⁷ × 111
Pull out a factor of 5 and multiply it with 111:
… = 2⁷ × 5⁶ × 5 × 111
… = 2⁷ × 5⁶ × 555
So 555 divides 10⁹ + 10⁸ + 10⁷.
