设梯形的高为h 原来梯形的面积=1/2(9+11)h
新梯形的面积=1/2(9+4+11+6)h
则1/2(9+4+11+6)h-1/2(9+11)h=15
解得:h=3
原来梯形的面积=1/2(9+11)h=30