First find the amount of dye each blouse takes: 20 dots/blouse * 10 ml/dot = <<20*10=200>>200 ml/blouse
Then multiply the dye used per blouse by the number of blouses to find the total amount of dye used: 200 ml/blouse * 100 blouses = <<200*100=20000>>20000 ml
Then divide the amount of dye needed by the amount of dye per bottle to find the number of bottles needed: 20000 ml / 400 ml/bottle = <<20000/400=50>>50 bottles
#### 50