First off #1 is the same as #2 and #3 so you sould be able to solve #2 and #3 after seeing how to do #1. #4 and #5 are the same type of problem, i will do #5.
1. The length of a rectangle is 4 times the width. The perimeter is 120cm. Find the width and the length of the rectangle.
Substitute and solve
5. The first side of a triangle is 6m shorter than the second side. The third side is 3 times as long as the first side. The perimeter of the triangle is 206m. Find the length of each side.
x is first, y is second, and z is third 
Use substitution and solve for y first
simplify by adding like terms
add
add
divide
solve the other equations with y=46
then
First side=
second side=
third side=
6. The third side of an isosceles triangle is 20m shorter than twice the length of the congruent side. The perimeter is 240m. Find the length of each side.
We know that an isosceles triangle has 2 equal sides
so the first and second side are = to each other.
so if P=x+y+z and x=y then P=x+x+z=2x+z if P=240 AND z=2x-20 then:
add
add
divide
so 
