Step-by-step explanation:
2 digit numbers are beetween 10 and 99
We are looking for numbers in wich the unit digits is less than the ten digit
we can make notice something about this numbers
0 is the smallest postive integer so all these numbers satisfy the condition
so we have now 9 numbers
here is another remark :
- 0 < 1 < 2 < 3 <4 <5 <6 < 7 < 8 < 9
numbers from 91 to 99 satisfy the condition except 99
so here are 8 other numbers
numbers from 80 to 89 satisfy the condition except 88 and 89
so here are 7 other numbers
the numbers that satisfy the keeps decreasing by 1 each time
- 8 (91⇒98)
- 7 81⇒88 + 80
- 6 71⇒76 + 70
- 5 61⇒ 65 +60
- 4 51⇒ 54 + 50
- 3 41⇒ 43 + 40
- 2 31⇒32 + 30
- 1 21 + 20
so the total number of numbers that satisfy the condition
9+8+7+6+5+4+3+2+1= 45
