(2)7÷50×400=56(6 points)

(3) Let two students with a score of 79.5 ~ 89.5 be A and B respectively.

The two students of 89.5 ~ 100.5 are C and D.

Tree diagram: (9 points)

* * * There are 12 possible results.

Among them, the two selected students scored no less than 90 points in the preliminary round.

2 (CD, DC)

So P (both students are not less than 90 points) = 212 =16 (12 points).