Re: A city's telephone numbers are made up of 6 digits. Knowing that the
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26 Jan 2021, 06:36
The question basically asks:
(Total number of possible combinations if you have 7 slots) - (Total number of possible combinations if you have 6 slots).
Number of available digits: 10 (0-9)
Restriction: First number cannot be 0
When we have 7 slots, i.e., __.__.__.__.__.__.__ (Visualization always helps)
Using Fundamental Counting Principal, we have \(9*10*10*10*10*10*10\) = \(9*10^6\) possibilities
Similarly when we have 6 slots, we have \(9*10^5\) possibilities
Difference:
(\(9*10^6\)) - (\(9*10^5\)) = (\(9*10^5\))(10-1) = (\(9*10^5\))(9) = \(81*10^5\)
Answer: D
Kindly correct me if I'm wrong. Thanks!