Mergers' affect on P/E

Hello Sir can you please explain how exactly do we calculate no. of shares in this ques? and what exactly is happening it's kind of confusing.... Peter plc has made an offer of one of its shares for every three of Baker plc. Synergistic benefits from the merger would result in an increase in after-tax earnings of $4m per annum. Extracts from the latest accounts of both companies are as follows: ............................................Peter plc .... Baker plc Profit after tax.................... $120m............. $35m Number of shares.............. 400 million..... 90 million Market price of shares ........250p ..............120p Assume that the price of Peter plc's shares rises by 50c after the merger and that Peter issues new shares as consideration. What will be the price-earnings ratio of the group?