{ diff(x1, t) = 1*k15*1/4*(1 - x3 - x1)/k15,
  diff(x2, t) = 1*k15*(6*x1/(1 + 2*x1)*(1 - x2)/(k14 + 1 - x2) - 3/2*x2/(k14 + x2))/k15,
  diff(x3, t) = 1*k15*(x2*(1 - x3)/(k14 + 1 - x3) - 7/10*x3/(k14 + x3))/k15 }