JAPAN ECONOMY SEEN GROWING 3.6 PCT IN 1987/88 Japan is expected to post a 3.6 pct rise in real gross national product in 1987/88, higher than the official 3.5 pct target, a private economic institute said. The Research Institute on National Economy said in a report the economy will start picking up in the April-June quarter, partly because of an improvement in earnings performance and capital spending in manufacturing industries. The institute assumed an average exchange rate in the year starting April 1 of 150 yen to the dollar. It predicted the Bank of Japan will not change the official discount rate in the year. The institute forecast that Japan's exports will gradually rise in the year in volume terms as the dollar's fall in the past 18 months is likely to help prop up the U.S. Economy. Japan's trade surplus is expected to narrow slightly to 90.2 billion dlrs in 1987/88 ending March 31 from an estimated 98 billion in the current fiscal year, it said.