15. Let's number some bars and consider bar #2 (see the fig.). On the left side of this bar there are 1 bar, 2 cubes, 1 pyramid and the right side there are 1 bar, 1 cube, 1 ball and 1 pyramid. It means that the cube and the ball have the same weight. Now, let's consider bar #1. Now we observe, that the weight of the pyramid equals to the sum of the weights of a cube and a ball. Therefore by the above mentioned, the pyramid is in equilibrium with 2 balls. So, bar #5 will be in equilibrium if instead of ? we put a pyramid, and then we easily check that the bar #3, #4 also will be in equilibrium.