Perimeter of a rectangle with a semicircle

A semicircle is joined to a rectangle. The rectangle is 20 centimetres long and 14 centimetres wide, and they join along one of the shorter sides. Here's the rectangle. And the semicircle goes on this end. Find the perimeter of the whole figure. Take pi as 22 over 7. Try it first. Pause if you like. Perimeter is the whole way round. So let's go round it. The curve, then the long side, then the bottom, then the other long side. And we're back where we started. That's the whole way round. Now look at this edge here, where the two shapes meet. We never went along it. This edge isn't part of the perimeter of the whole figure. The curve first, on its own line. That's how the working goes: every curve first, then the straight edges. The semicircle sits on the 14 centimetre side, so 14 is its diameter. So the arc is a half, times pi, times the diameter. Half, times 22 over 7, times 14. Cross out the 2 and the 14. 14 divided by 2 is 7. Then cross out the two 7s. 7 divided by 7 is 1. That leaves 22 centimetres. Now the straight edges, in one line. Down the long side, 20. Along the bottom, 14. And back up the other long side, 20. 22, and 20, and 14, and 20. That's 76 centimetres. If you got 90, here's what happened. The 14 centimetres inside was added. When two shapes stick together, we only count the sides on the outside. The perimeter is the sides on the outside. The curve and the three straight sides are all counted. The edge where the two shapes meet is inside the figure. So it is not counted.