/sci/ - Science & Math

>>15213744
d1d2d3 = 10*10*10 = 1000.
d1d2d3 = d4d5d6 --> 1000 = d4d5d6.
d1d2d3 = d5d6d7 --> 1000 = d5d6d7.

1000 + 1000 = 2000 except we're double counting the cases where d4d5d6 = d5d6d7 which is fake and gay.

d4d5d6 = d5d6d7 --> d4 = d7 = 10.

so we subtract the overlap 2000 - 10 to get the answer 1990 memorable telephone numbers.