Explanation:
6r7₈ = 511₉
6 x 8² + r x 8¹ + 7 x 8^o = 5 x 9² + 1 x 9¹ + 1 x 9^o
6 x 64 + 8r + 7 x 1 = 5 x 81 + 9 + 1 x 1
384 + 8r + 7 = 405 + 9 + 1
391 + 8r = 24
r = \(\frac{24}{8}\)
= 3