Hello Bing,
There's many ways to solve this (including sequences), but we can divide 1000 by 7 and use the remainder to figure out the column. Why division by 7? Because there are 7 columns.
Since you're dividing by 7, the remainder will be any integer from 0 to 6.
If the remainder is 0, that means 1000 goes under the G columns with 7, 14, 21 since they are multiples of 7, and dividing any multiple of 7 by 7 will give you a remainder of 0.
If remainder is 1, 1000 goes under the A column with 1, 8, 15.
Let's divide: 1000/7 = 142, remainder of 6.
That means 1000 will appear in the column of 6, which is F.
