Orange = <<4.5=4.5>>4.5 L
Cherry = 2 * 4.5 = <<2*4.5=9>>9 L
Apple = 9 - 1.5 = <<9-1.5=7.5>>7.5 L
Total punch = 4.5 + 9 + 7.5 = <<4.5+9+7.5=21>>21 L
They had 21 liters of fruit punch for the picnic.
#### 21