(1) The tree diagram is as follows: (Those lines are not easy to draw! )

The first box: A B C D E

The second box: abcde abcde abcde abcde abcde abcde abcde abcde abcde abcde abcde

(2) According to the table, the probability of "at least one blank card" is 0.75.

(3) According to the meaning of the question:

The first box * * * has (5+x) cards.

The second box * * * has (5+x) cards.

So * * * has (5+x)? One possibility is that,

"At least one blank card" * * Yes (10x+x? ) possibility

Then:

( 10x+x？ )

————=0.75

(5+x)？

To simplify: x? -20x-75=0

Solution: (There may be some mistakes in the calculation process ...)