"twice a number divided by 5"

2x/5

Twice suggests multiplication, so you would put 2 for twice.

We don't know the number, so we put a variable, x.

Divided by 5, means you would divide 2x by 5.

i do not know this sorry : (

4 and 5)16.97 or 12√2

6)15.59 or 9√3

7)y = 15.59 or 9√3     x =27

8)17.80