each 10 has 1, 7
1000-10 = 900 will have 900/10 = 90 sevens
170 till 179 will have 10 additional 10 sevens. so will 270-279 and so on, that will make 10 x 9 = 90 sevens
700 till 799 will have additional 100 sevens
so total 90 + 90 + 100 = 280 sevens.