They already have 20 tubs in storage, so they only need to buy 100 needed tubs - 20 tubs in storage = <<100-20=80>>80 tubs.
From the new vendor, they buy 80 tubs / 4 = <<80/4=20>>20 tubs.
They now have a total of 20 + 20 = <<20+20=40>>40 tubs.
They are therefore going to buy 100 needed tubs - 40 owned tubs = <<100-40=60>>60 tubs from the usual vendor.
#### 60