This drop was caused by a combination of sharply lower petroleum prices in world markets and voluntary production cuts in Brunei. The GDP recovered somewhat since 1986, growing by 12% in 1987, 1% in 1988, and 9% in 1989. In recent years, GDP growth was 3.5% in 1996, 4.0% in 1997, 1.0% in 1998, and an estimated 2.5% in 1999. However, the 1999 GDP was still only about $4.5 billion, well below the 1980 peak.. How many percentage did 1997 and 1998 have combined?