Pendulum problem helpfinding weight

[jmb07]

Martin caught a fish and wanted to know how much it weighed, but he didnt have a scale. He did however have a stopwatch, a spring, and a 4.6 N weight. He attached the weight to the spring and found that the spring would oscillate 20 times in 65 s. Next, he hung the fish on the spring and found that it took 220 s for the spring to oscillate 20 times. What is the weight of the fish?

I know that omega=sqaure root of (k/m)
However, I just have no idea how to start or apply this equation to solve this problem.

 

[mgb_phys]

Apply the equation to both cases
You know K is the same
You know the frequency in both cases
So you can write two equations with only one unknown - the second mass

 

[CompuChip]

This is a typical spring question. Instead of telling you the spring constant directly they give you some information from which you can first derive k. Then you can reduce the problem to the basic "He hangs the fish on a spring with spring constant <fill in number you just calculated> N/m and found that it took 220 s for the spring to oscillate 20 times. What is the weight of the fish?"