They sell twice as much raspberry jelly as plum jelly, so they sold 2 * 6 = <<2*6=12>>12 jars of raspberry jelly today.
The raspberry jelly sells a third as much as the grape jelly, so they sold 12 * 3 = <<12*3=36>>36 jars of grape jelly today.
The grape jelly sells twice as much as the strawberry jelly, so they sold 36 / 2 = <<36/2=18>>18 jars of strawberry jelly today.
#### 18