số số có 4 chữ số là : (9999 - 1000) + 1 = 9000 ( số)
Số số có 4 chữ số chia hết cho 5 là: (9995 - 1000) : 5 + 1 = 1800 ( số)
Số số có bốn chữ số không chia hết cho 5 là : 9000 - 1800 = 7200 ( số)
khó quá
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dãy số có 4 chữ số chia hết cho 5 là : 1000, 1005 ,..., 9995
số số hạng : (9995-1000) : 5+1= 1800 số
từ 1000 đến 9999 có 9000 số
số có 4 chữ số ko chia hết cho 5 là : 9000-1800=720 số
số có 4 chữ số từ 1000 đến 9999 có 9999-1000+1
