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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
hãy cho biết có tất cả bao nhiêu số có hai chữ số mà hai chữ số của mỗi số đó hơn kém nhau 6 đơn vị?
9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999
co bao nhieu so co 2 chu so ma 2 so do kem nhau hon 7 don vi
Ta có:3.3=9<10
4.4=16>10
9.9=81<100
10.10=100>99
Vậy có tất cả :
(9-4):1+1=6(số thỏa mãn)
Hàng Đơn vị có 3 cách chọn :1:4:9
Hàng Chục có 3 cách chọn :1:4:9
vậy có :3x3=9(số)