các số có hai chữ số mà tích hai chữ số nhỏ hơn 3 là:

10,11,12,20,21,30,40,50,60,70,80,90.

vậy tất cả có 12 số

10 , 11 , 12 ,20, 

các số có hai chữ số mà tổng hai chữ số không bé hơn 15 là:

69,96,78,87,79,97,88,89,98,99

vậy tất cả có 10 số.

có 15 số

9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999

9 - 3  = 6 vậy có các số: 93; 39

8 - 2 = 6 vậy có các số: 82; 28

7 - `1 = 6 vậy có các số: 71; 17

6 - 0 = 6 Vậy có số 60

   Có tất cả 7 số có hai chữ số mà hai chữ số của số đó hơn kém nhau 6 đơn vị.

co bao nhieu so co 2 chu so ma 2 so do kem nhau hon 7 don vi

có 15 số