Systems of Differential Equations · Superchallenge problem

Four tanks, each holding $100$ L of water, are joined in a ring: well-mixed water flows at $100$ L/min from tank 1 to tank 2, from tank 2 to tank 3, from tank 3 to tank 4, and from tank 4 back to tank 1. At $t=0$ (minutes) tank 1 holds $4$ kg of dissolved salt and the others hold none. Find the amount of salt, in kg, in tank 3 at the first moment when tanks 1 and 3 hold equal amounts.