1-20, 25, 30-100
Use the calculator and column buttons to create a boolean formula (or type one in directly), indicating which rows should be deleted. All the rows that satisfy the condition (expression evaluates to TRUE, or not equal to zero) will be deleted.
When the "Delete" button (or <Return> key) is pressed, fv will ask for confirmation before actually deleting the rows.
See the calculator help page for information on how to specify the conditional expression.