coffee machine is installed in a students' common room. It dispenses white coffee by first releasing a quantity of black coffee, normally distributed with mean 122.5 ml and standard deviation 7.5 ml, and then adding a quantity of milk, normally distributed with mean 30 ml and standard deviation 5 ml. Each cup is marked to a level of 137.5 ml and if this level is not attained the customer receives the drink free of charge. What percentage of cups of white coffee will be given free of charge?