plural/singular question

Question

Can I use singular verb for these sentences?

Sentence 1. "Twenty gallons of paint is needed to paint the house"

Sentence 2. "Three members of the staff/crew is needed to paint the house"

Note

I know there are other threads with similar questions so plaese don't mark this as duplicate as I'm asking a particular questions in this thread.

Edit. I'm in cofusion because the first example has a mass/uncountable noun and the second example has a singular noun and from what it says here it should be singular verb because it says the following: "Generally speaking - though there are exceptions - the fraction takes the same plurality as the thing it modifies; see here for a good explanation." so I don´t know if the sentences need a singular or plural verb.

Answer 1

1. Twenty gallons of paint is needed to paint the house.

is fine. Twenty gallons of paint would be considered a singular unit, as you said. However:

Twenty one gallon cans of paint are needed to paint the house.

is clearly plural.

2. Three members of the crew are need to paint the house.

Even though you are referring to part of the crew, the number in the example is three, so I would not use the fraction rule. You could use a fraction though:

Half of the members of the crew is needed to paint the house.

half is a fixed group and has a set value based on the size of the crew.

Answer 2

In the first, I would probably say "are", but I would accept "is" as perfectly grammatical, treating "twenty gallons of" as a quantifier on the non-countable noun "paint".

In the second, I would accept only "are", because "Three members of" cannot be a quantifier on "crew" or "staff". If you said "The three-strong crew" I would accept "is", though I would still say "are" myself.