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Q: Two digit numbers contain exactly one 4?

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Any pair of digits (not including 0), can be used to generate 14 four-digit numbers. If one of the digits is 0, only seven will start with a non-zero digit.

There are only two prime numbers that are consecutive numbers, 2 and 3. Their product is 2 x 3 = 6. The first prime numbers are 2, 3, 5, and 7 and the only two consecutive prime numbers whose product is a single digit are 2 and 3. (The next two consecutive prime numbers, 3 and 5, have a two-digit product.)

679 is the product of the largest single-digit prime number and the largest two-digit prime number.

If two digit number can have a repeated digit (eg 44) then there are 20 possible numbers, otherwise there are 16 possible numbers: There are 4 single digit numbers; With repeats allowed there are a further 4 x 4 = 16 two digit numbers making a total of 20; Without repeats allowed there are a further 4 x 3 = 12 two digit numbers making a total of 16.

The one that is prime.

Related questions

There are 6804 such numbers.

13

18 of them.

There are 90 two digit numbers, 10 through 99. There are 18 two digit numbers containing a 9. They are 19, 29, 39, 49, 59, 69, 79, 89, and 90 through 99. Therefore, there are 72 two digit numbers that do not contain a 9.

25 and 49

Excluding numbers with leading 0s, there are 6804 such numbers.

Leaving out 077, since it is actually a two digit number, the answer is 35.

27: 077,177,277,377,477,577,677,877,977,717,727,737,747,757,767,777,787,797

18 positive integers and 36 integers (negative and positive)

There are 21 two-digit prime numbers.

There are 90 two-digit numbers... starting with 10 and ending with 99.

what is the least possible sum of two 4-digit numbers?what is the least possible sum of two 4-digit numbers?

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