Question 205489: How many two digit numbers are there whose first digit is larger than the second digit?
Answer by RAY100(1637)   (Show Source):
You can put this solution on YOUR website! Visualize the 100"s chart,,,1-9,,10-19,,20-29,,etc
.
let's start at the 90's row,,,100 is 3 digits so it doesn't count
90,91,92,93,94,95,96,97,98,,(but not 99) all have less, net 9
.
The 80's row,,,80,81,82,83,84,85,86,87,(but not 88,89),,,net 8
.
The 70's row,,,70,71,72,73,74,75,76,,(but not 77,78,79),,,net 7
.
The 60's row,,60,61,62,63,64,65,( but not 66,67,68,69),,net 6
,
The 50's row,,,50,51,52,53,54,(but not 55,56,57,58,59),,net 5
.
The 40's row,,,40,41,42,43,(but not 44,45,46,47,48,49),,,net 4
.
The 30,s row,,,30,31,32,(but not 33,34,35,36,37,38,39(,,,net 3
.
The 20's row,,,20,21,(but not 22,23,24,25,26,27,28,29),,net 2
.
The 10's row,,,10,(but not 11,12,13,14,15,16,17,18,19),,,net 1
.
The 1's row,,,,none because all single digits,,,,net 0
.
Adding all of those available (9+8+7+6+5+4+3+2+1) = 45
.
of interest, total numbers is 99-9=90,,,therefore half have greater and half equal or less digits.
|
|
|
