P, Q, R and S are four places. To travel from P to R, one travels via Q and to travel from P to S, one travels via both Q and R. Between P and Q, there are 3 routes; between Q and R, there are 4 routes; and between R and S, there are 5 routes. The time taken to travel through these routes is exhibited on the following chart in hours for a specific vehicle : [CHART — columns: Routes | Between P and Q | Between Q and R | Between R and S; Route 1: 2, 1, 2; Route 2: 3, 1·5, 2; Route 3: 4, 2, 2·5; Route 4: —, 2, 3; Route 5: —, —, 3·5] How many possible routes are there if the journey takes exactly 7 hours?