A039435 Numbers n such that representation in base 10 has same nonzero number of 7's and 8's.

78, 87, 178, 187, 278, 287, 378, 387, 478, 487, 578, 587, 678, 687, 708, 718, 728, 738, 748, 758, 768, 780, 781, 782, 783, 784, 785, 786, 789, 798, 807, 817, 827, 837, 847, 857, 867, 870, 871, 872, 873, 874, 875, 876, 879, 897, 978, 987, 1078, 1087