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The correct answer is 10*9/2 = 45. The 45 are:
01, 02, 03, 04, 05, 06, 07, 08, 09
12, 13, 14, 15, 16, 17, 18, 19,
23, 24, 25, 26, 27, 28, 29,
34, 35, 36, 37, 38, 39,
45, 46, 47, 48, 49,
56, 57, 58, 59,
67, 68, 69
78, 79,
89.
In a combination, the order of the elements DOES NOT MATTER.
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How many 4 number combinations are there in 4 numbers?
-- 4 numbers can be lined up in 24 different ways-- If the order doesn't matter, then there's only one4-number combination of 4 numbers.
How many 5 number combination are there for numbers 1 - 39 when one of the numbers is 9?
277612920, but only if any number (including 9) can appear more than once.
How many numbers in a billion?
a billion numbers
How many 4 digit numbers can you write using only the digits 2 and 5?
With each digit having only 2 possibilities, the answer is 2 to the 4th power, which is 16. The 4 is because there are 4 digits. Think about the binary numbers 0000 to 1111, there are 2 possibilities for each digit. If your constraint is that the digits must have at least one 2 and at least one 5, then eliminate the two combinations 2222 and 5555, and that answer would be 14.
What is the mean between these 1.2 1.3 1.4 1.5?
To find the mean of a set of numbers, you add all the numbers together and then divide by the total number of values. In this case, the sum of 1.2, 1.3, 1.4, and 1.5 is 5.4. Dividing 5.4 by 4 (the total number of values) gives you a mean of 1.35.
