Suppose: there are x students participating in the computer interest group, then

3*x- 13=53

The solution is x=22.

Twenty-two students joined the computer interest group.

The third question: Find the height of this trapezoid.

Suppose: the height of this trapezoid is x meters.

(35+65)* x/2=300

The solution is x=6.

The shortest path is 6 meters.

PS: Little friend, think about these questions yourself next time, and don't always want to find someone else to answer them ~, hehe!