please help with questions 4, 5 & 6. thank you

4. If a wave travels at 4.0 m/s in one spring, that is 3.0 m long and then at 5.0 m/s in the next spring, which is 2.0 m long, how long will a pulse take to travel the lengths of the two springs? 5. A standing wave pattern is created between two discs that reflect radio frequencies. If the distance between two consecutive nodes is 0.27 m, what is the frequency of the wave source? (Use c=3.00X10 m/s) 6. A transmission cable is run from Barrie to Angus (about 20 km). One day the service stops working and your boss suspects that the cable has been cut. To find the location of the damage, you hook up a pulse generator and discover that the return pulse is not inverted and arrives back at the source 83 us later. If the speed that electrical signals travel in the cable is 1.80X10 m/s determine the distance to the trouble and if the cable is open or shorted