t0 = 4;
tk = 80;
tp = 105;
cvody = 4186;
Δ = tk  -  t0;
m = 200*100;
Q = 
        m*cvody*(Δ/(24*60*60)) // 
          N;
Print["Q= ", Q, "  W"]

Δ1 = tp - t0;
Δ2 = tp - tk;
Solve[
        Q == 
          kS*((Δ1 - Δ2)/Log)[Δ(1/Δ)2]];
\(Print["kS= ", 
      kS /. %[\([1]\)], \*""  W.\!\(K\^\(-1\)\)""]\)