How many seven digit numbers contain the number seven at least once?

Answer 1

To find the total number of seven-digit numbers that contain the number seven at least once, we can use the principle of complementary counting. There are a total of 9,999,999 seven-digit numbers in total. To find the number of seven-digit numbers that do not contain the number seven, we can count the number of choices for each digit (excluding seven), which is 9 choices for each digit. Therefore, there are 9^7 seven-digit numbers that do not contain the number seven. Subtracting this from the total number of seven-digit numbers gives us the number of seven-digit numbers that contain the number seven at least once.

Answer 2

4,748,472

Confirmed using the following C# function

string sTemp;

int total = 0;

for (int i = 1000000; i < 10000000; i++)

{

sTemp = i.ToString();

if (sTemp.Contains("7"))

{

total++;

}

}

label1.Text = total.ToString();

Answer 3

How many seven-digit numbers contain the digit '7' at least once?