The letters D, E, G, J, N stand in a series in which the step from one letter to the next grows longer every time. Which letter comes after N?

D to E is one place, E to G two, G to J three, J to N four, so the next step is five. N stands at place 14, and 14 + 5 = 19, which is S. Repeating the step of four instead of enlarging it gives R; enlarging it to six gives T; keeping the step at three gives Q.