To find out how many buckets are needed to fill a 20-liter tub with a 4-liter bucket, you can divide the total volume of the tub by the volume of the bucket.
\[ \text{Number of buckets} = \frac{\text{Volume of tub}}{\text{Volume of bucket}} = \frac{20 , \text{L}}{4 , \text{L}} = 5 \]
You would need 5 buckets of water to fill the 20-liter tub.