At a bottling plant, two machines are used to ll 500 ml bottles of pop

At a bottling plant, two machines are used to ll 500 ml bottles of pop. The amount of pop put into each bottle by Machine A is a normal random variable with a mean of 508ml and a standard deviation of 5ml. Similarly, amount of pop put into each bottle by by Machine B is a normal random variable with a mean of 506ml and a standard deviation of 4ml. If Machine A is used to ll 60% of bottles. (a) What percentage of bottles containing less than 499ml comes from Machine B Can anyone explain this to me ?