1. Which units are used to measure perimeters?

The units used to measure the perimeter of a shape are the same as the unit for the sides of the shape. 
i.e. Units of length. For example, metres or centimetres.

2. What is the perimeter of a rectangle?

The perimeter of a rectangle is the sum of all of its sides.
Because the opposite sides of a rectangle are equal there is a formula.

If the length is L and the width is W then:

Perimeter = 2 x length + 2 x width = 2L + 2W

3. If one measurement is in one type of unit and the other in a different unit how is the perimeter worked out?

Before adding the lengths, both measurements must be in the same units.

e.g. If the length of a rectangle is given as 1 metre and the width as 50 centimetres, one way would be to change 1 metre into 100 centimetres.
Then the perimeter can be found by calculating 2 × 100 + 2 x 50 which equals 300 cm which could then be changed back to 3 metres.