Building number memory retention. (exercises)

To become a good sudoku number player you must be able to "hold" a number(s) in your mind.

Hard1 Three-Grid Excercise

Below are three 1 to 9 number grids. Each grid is missing two numbers - determine the missing numbers - making a mental note of them.
7
4
2
5
3
9
1
3
8
9
7
5
6
4
9
5
1
4
3
2
8

Eg. grid one: 6 and 8
      grid two: 1 and 2
      grid three: 6 and 7

Now proceed to the three below grids. Determine the missing numbers in each - making a mental note of them, also.

4
6
2
3
8
5
7
8
3
6
7
1
2
4
9
2
1
7
5
4
9
8
6

Now recite all the above grid numbers. Repeat this process until you can. (mentally repeating and recalling the numbers)

Ok, once able to do the above - proceed and determine the missing numbers in the three below grids.

1
8
9
7
5
6
3
4
2
6
1
3
7
8
3
7
5
6
8
1
2

Ok, now recite all the above determined missing grid numbers. Repeat this entire process on this page until you can.

Can you recall all the below missing numbers?

7
4
2
5
3
9
1
3
8
9
7
5
6
4
9
5
1
4
3
2
8
4
6
2
3
8
5
7
8
3
6
7
1
2
4
9
2
1
7
5
4
9
8
6
1
8
9
7
5
6
3
4
2
6
1
3
7
8
3
7
5
6
8
1
2

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