function p5_90
  a=4.2e-6; dx=.125/12
  dt=12; k=.48; hi=1.2; ho=2.6;
  r=a*dt/dx^2; To=35; T1o=35; T2o=35; T3o=35; T4o=35;
  error=1;
  i=0;
  while error>1e-4
      i=i+1;
      Tinfi=To+2*(dt*i)/60;
      if Tinfi>72
          Tinfi=72;
      end
      T1n=(1/3)*(2*r*T2o+(3-2*r-2*r*dx*hi/k)*T1o+(2*r*dx*hi/k)*Tinfi);
      T2n=r*T3o+(1-2*r)*T2o+r*T1o;
      T3n=r*T4o+(1-2*r)*T3o+r*T2o;
      T4n=(1/3)*((2*r*dx*ho/k)*To+(3-2*r-2*r*dx*ho/k)*T4o+2*r*T3o);
      time=i*dt/60;
      error=abs(T1n-T1o);
      T1o=T1n; T2o=T2n; T3o=T3n; T4o=T4n;
  end
  disp('    T1        T2        T3        T4')
  disp([T1n T2n T3n T4n]);
  disp('Time min');
  disp(time);