Assuming that no one should copy 450 copies, the cooperation will copy 900 copies, divided by 10 hour, and copy 90 copies per hour.

According to work efficiency, Fangfang makes 40 copies per hour and Hui Hui makes 50 copies per hour.

So Fangfang finished 450 copies, which took 1 1.25 hours.

Hui Hui only needs 9 hours to finish, so Fangfang spent 2.25 hours more than Hui Hui!

In other words, if Fangfang's speed is X, then Hui Hui's speed is 5/4X, and when they cooperate, their speed is 9/4X.

Assuming that the number of cards that each person needs to make is A, then 2A= 10*9/4X. X=4/45A，

So the time for Fangfang to finish A is 45/4 hours, which is 1 1.25 hours.

Hui Hui's speed is 5/4X=5/45A, so the time to complete A is 45/5h, which is 9 hours!

So the answer is 2.25 hours!