/sci/ - Science & Math

>>15213744
first 3 numbers: each number can go from 0 to 9, so 10 possible choices each = 1000 in total

We have 2 chains: abc-Xabc and abc-abcX
since abc can be all 1000 numbers from 000-999, we pick X to go from 0 to 9, each step we count 1000 numbers, for both chains

So we get 10000 possible numbers of the form abc-Xabc and also 10000 possible numbers of the form abc-abcX, however, in the case where every number is the same (000-0000) we are counting that same number twice in both chains, therefore we just need to subtract 10 to the total.

the total number of different memorable telephone numbers is 19990.

I hope this is correct.