Physics 101 wave interference?

An instructor gives a demonstration in which he makes a standing wave on a long thin slinky. The slinky is 5 meters long. If you see 3 loops on the slinky when the instructor shakes the end of the slinky at a rate of 2.5 cycles per second, then the speed of transverse waves on the slinky is

(Answer in m/s)

I know the answer is NOT 6.25 m/s, even though I have found that answer on the internet. My online homework says that is wrong. I'm thinking the answer is 12.5 from the formula speed= frequency x wavelength but im not sure. Can someone help me out please?!


I also tried the answer 4.166 and that was wrong too :-(

The answer is 8.325 m/s. you take 3 loops divided by 2--> 3/2 equals 1.5m. Then you divide 1.5m into 5 meters of slinky--> 5m/1.5 = 3.33m. Then because of the equation of speed, V(speed)=wavelength x frequency. Our wavelength is 3.33m, and the frequency is the HZ (remember cycles per second?). so therefore, speed= 3.33m * 2.5 HZ. The answer then becomes 8.325 m/s.