SOLUTION: if i have to pick two numbers from 1 to 10, provided the first number can't be greater than the second number. How many possible way I can represent the two number. Please suggest

Question 563661: if i have to pick two numbers from 1 to 10, provided the first number can't be greater than the second number. How many possible way I can represent the two number. Please suggest the formula to use?
E.g.
12 - valid
21 - not valid
Answer by josmiceli(19441) (Show Source): You can put this solution on YOUR website!
If the 1st number is 1, the 2nd can be 2-10
If the 1st number is 2, the 2nd can be 3-10
If the 1st number is 3, the 2nd can be 4-10
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This progression is

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A formula would be if you can choose numbers
from 1 to n, then the possible arrangements of 2 numbers
the 1st being smaller than the 2nd
is (n-1) + (n-2) + (n-3) + . . . + 1
This sum is
check:
for 1 to 10,

OK

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