Parallelism Answer 4

4. Barb studies each problem carefully and works out each one.

One can not have a compound verb construction that does not include a modal for the first verb and then turn around and include a modal with the second verb.

Variant answer:

4. Barb can study each problem carefully and work out each one.

Here the modal is included in front of the first verb, but it is not included in front of the second verb. This is allowed; furthermore, it is inferred that the modal also exists in front of the second verb too.