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How many 4-digit numbers have the last digit equal to the sum of the first two digits?
450.
(Note: "x" below signifies that this digit is temporarily being ignored)
This is true for:
10x1
11x2
12x3
13x4
14x5
15x6
16x7
17x8
18x9
So for the first eligible 1000 numbers there are 9 (when we disregard the third digit).
For the next 1000 (that is 2000 to 2999) there will be 8 (it is one less because the first digit is 2 and not 1)
This pattern will continue all the way through the 4 digit numbers, so that our answer is:
9+8+7+6+5+4+3+2+1 = 45.
However as "x" above (that we temporarily ignored) can be any digit we have to multiply this by 10 (as there are 10 digits). Therefore the answer will be 450!
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How many three digit numbers have the sum of their digits equal to 22?
21.
What two digit numbers whose two digits added together equal 42 and multiplied together equal 82?
Two (or four) digits added together cannot equal 42. Two-digit numbers multiplied together cannot equal 82.
How many three digit numbers is average of three digits equal to 2?
21
How many integers between 200 and 300 have the sum of their digits equal to 15?
Since the first integer of the 3-digit numbers must be 2, the sum of the second and the third digits would be 13.Since the largest digit is 9, the 3-digit numbers could be 249, 294, 258, 285, 267, and 276.Thus, there are 6 integers between 200 and 300 whose sum of their digits is 15.
What are Two nine digit numbers divided to equal pi?
No two numbers that can be completely written down with digits can be added, subtracted, multiplied, or divided to equal PI. If they could be divided to equal PI, then PI would be a rational number. But it isn't.
