Is plastic lighter than metal

Answers:
This depends on several factors. First what plastic and what metal are you asking about? Generally speaking, most plastics used are less dense, more flexible and less hard than steel in modern products. This can translate to a savings in weight and an increased resistance to striking or falling damage at the cost of more bulk (Space).
However, when bulk AND weight is important such as in a laptop case, some metals like aluminum or titanium can get the job done with less weight while being thinner but be much more expensive to produce. It's all about tradeoffs.

An aluminum drinking cup where strength isn't its primary requirement, can be made lighter and thinner than a similar cup made of polycarbonate. However the aluminum cup will be less resistant to damage if dropped or crushed.

So it really depends upon the intended application and which type of metal or plastic you are talking about.
ad
Relevant answers:

Can you answer these questions?