How many ten-digit telephone numbers are possible if the first three digits must be different? The answer is 72,000,000,000.... I need the method to get that:)

Answer 1

They must assume that the lead digit could be a zero
so first 3 digits : 10 x 9 x 8 = 720
That leaves 7 more digits where anything goes
that would be 10^7
so final number = 720 x 10^7
= 7 200 000 000

I disagree with their answer.

Answer 2

Ohh i accidently added an extra zero...thank you:)