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**1. The problem statement, all variables and given/known data**

Four penguins are being playfully pulled along frictionless ice by a curator. The masses of three penguins and the tension in two of the cords are m1 = 12 kg, m3 = 15 kg, m4 = 20 kg, t2 = 111 N, and t4 = 222 N. Find the mass of the second penguin.

Note: t2 is the tension between the second penguin and the third penguin, and t4 is the tension between the curator and the fourth penguin.

**2. Relevant equations**

F=MA

**3. The attempt at a solution**

I don't know how to set up this problem. It is useless to separate it into its components using F=MA, since the acceleration is not given nor can it be derived without the second mass. I don't really understand the tension force very well since it runs counterintuitive to my sense of a free-body diagram. Can anyone clue me in on how to start?