a number like 123 or 479 is called a rising number. That's because each digit in the number is greater than the digit to its immediate left. how many 3-digit rising numbers are there? show all ur work. it is hard but try . thank u.
2007-10-12 12:19:37 · 4 answers · asked by se 1 in Science & Mathematics ➔ Mathematics
20's : 23, 24, 25, 26, 27, 28, 29......7
30's: 34, 35, 36, 37, 38, 39.......6
see a pattern
40's: 5
50's: 4
60's: 3
70's: 2
80's: 1
Now all 3 digit numbers that begin with 1, will include all of these, 7+6+.....+1 = 28
all 3 digit numbers that begin with 2, will include 30's and above = 21
Notice a pattern?
28 + 21 + (21 - 6) + (21-6-5) + .....
=84
2007-10-12 12:33:01 · answer #1 · answered by Anonymous · 1⤊ 0⤋
123-129, 134-139, 145-149, 156-159, 167-169, 178-179, 189, 234-239, 245-249, 256-259, 267-269, 278-279, 289, 345-349, 356-359, 367-369, 378-379, 389, 456-459, 467-469, 478-479,489, 567-569, 578-579, 589, 678-679, 689, 789
2007-10-12 19:31:18 · answer #2 · answered by Anonymous · 0⤊ 0⤋
84 of them
2007-10-12 19:34:39 · answer #3 · answered by Anonymous · 0⤊ 0⤋
₉C₆ = (9!)/[(6!)*(3!)] = 84
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2007-10-12 20:10:14 · answer #4 · answered by oregfiu 7 · 0⤊ 0⤋