A fruit grower can use two types of fertilizers in his garden , brand P and Q . The amounts (in kg ) of nitrogen , phosphoric acid , potash and chlorine in a bag of each brand are given in the table. Tests indicate that the garden needs atleast 240 kg of phosphoric acid , atleast 270 kg of potash and atmost 310 kg of chlorine. If the grower wants to minimise the amount of nitrogen added to the garden, how many bags of each brand should be used ? What is the minimum amount of nitrogen added in the garden ?