Each problem shows a labelled rectangle and asks for the distance round the outside. Add the two different sides and double, or add all four - both work, and seeing that they agree is worth as much as the answer.
Perimeter is usually taught after area and is the easier of the two, which surprises students who meet them in that order. It is addition, and the only trap is forgetting that a rectangle has four sides rather than two.
Add all four sides. Since opposite sides are equal, that is the same as adding the two different ones and doubling: a 9 by 3 rectangle has a perimeter of 2 × (9 + 3), which is 24 cm.
Because a perimeter is a length - the distance you would walk round the edge. Only area is measured in squares.
Yes, and it is worth noticing. A 1 by 11 rectangle and a 5 by 7 one both have a perimeter of 24 cm, but their areas are 11 cm² and 35 cm².
Third and fourth grade, usually alongside or just after area.