.
In total, there are 10,000,000 7-digit numbers, starting from 0000000 and completing by 9999999.
Of them, all starting from "0" are prohibited; in all, the amount of these numbers is 1,0000,000.
Next, the numbers starting from "1", are prohibited, too; the amount of these numbers is 1,000,000, again.
The numbers starting from 911 are prohibited; the amount of these numbers is 10,000.
The numbers starting from 411 are prohibited; the amount of these numbers is 10,000.
Therefore, to answer the question, we need subtract 1,000,000, then 1,000,000 again, then 10,000 and then 10,000 again, from 10,000,000.
It will give you the answer
10,000,000 - 1,000,000 - 1,000,000 - 10,000 - 10,000 = 7,980,000.
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Comment from student: Thank you so much! Your solution is the one that I got. But the answer on the test
answer key is 7*9*9*10*10*10*10. I cannot figure out how that is the correct answer. Any idea?
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My response :
I have two ideas.
My 1-st idea is that my solution is correct.
My 2-nd idea is an advise to you: do not accept any other answer.
Since if the answer is different, it means that the other solution is incorrect (wrong).
