It's already done! (+point H is located at the intersection of BC and FG extension lines)

1.AFG∽CHG (will be certified! ! ),

AF/CH=FG/GH (right! ! ),

Let CD=X meters,

CH=3-X，

AF/3-x =1.75/2.8-1.75 (you missed the length of af! )。

Find out that x is the length of the CD!

2. there is y,

{ab/y < 20 ①, (AE-CD)/y > 20 ② (y is obtained by solving). Ps: You will understand! ! )

Finding y is the number of steps!

(If there is any mistake, please forgive me! There shouldn't be! ! )