A rectangle of perimeter 45cm has sides of length x and x+4. Work out the value of x.

First, draw a diagram to help visualise the problem.
Next, write out the full sum: x + x + (x+4) + (x+4) = 45
This simplifies to: 2x + 2(x+4) = 45
Which expands to: 2x + 2x + 8 =45
This simplifies to: 4x + 8 = 45
Which can be rearranged to: 4x=37
Therefore, x = 37/4 = 9.25