An alloy contains copper and zinc in the ratio of 4 : 3 respectively. When 'T' kg of zinc is added to the alloy then the weight of copper in the alloy becomes 50% and when 20 kg of copper is added to the alloy then the weight of copper in the alloy becomes 60%. If x² - px - (T + 4) = 0 and the difference between the roots of the equation is 15 then find the value of the 'lowest' root of the equation? (Note : p > 0)