Imagine a train and two carriages as shown in this picture. The train is 10 meters long and Each carriage is 5 meters long.

There are two sidings. The Siding on the left can only hold one carriage. The Siding on the Right can only hold the engine and one carriage
Use the engine to exchange the positions of carriages A and B and return the engine to the same position it was at the start
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