7. Bav is counting the edges on a particular prism. The prism has more than 310 edges, it has fewer than 320 edges and its number of edges is odd. How many edges does the prism have?
7. 315 Suppose that the cross-section of the prism is an $N$-gon with $N$ edges. The prism will have $N$ edges in each of its 'end' faces and a further $N$ edges connecting corresponding vertices of the end faces. Therefore the number of edges is $3 N$ and hence is a multiple of 3. The only multiples of 3 in the given range are 312,315 and 318 . Since we know the total is odd, the prism has 315 edges.