The back of an envelope is drawn as a rectangle with both diagonals and a flap whose tip lies below the centre. How many triangles are there in the figure?

On the top side, with the apex at the centre, at either crossing of flap and diagonal, or at the flap tip: 4. Halves cut off by a diagonal: 4. Left, right and bottom triangles meeting at the centre: 3. Triangles with a top corner, a crossing of a flap edge with a diagonal, and the centre, a bottom corner or the flap tip: 6. Total $4+4+3+6=17$.