I have a grocery dotted list of pairs which are missing items that I need and I don't have any of them, so I am going to the supermarket.
Each pair in the dotted list is the name of the missing item that I need and a natural number which is the quantity/amount i.e. how many of this item I need.
I am in the supermarket now and I see an array of packages of items standing on shelf.
There are packages that don't contain items that I don't need and there are packages that contain items that I don't need.
I have enough money to buy all the packages, but if I will do this then I will get items that I don't need, so I don't want to do this.
Surely that I won't buy any package that contains an item that I don't need, but note that I don't want to have too many of any item that I need.
I can peek inside each package to see what items it contains and how many of that item it contains.
I cannot buy partial package, i.e. I cannot for each package decide which items from the package I buy and which I don't buy.
My only choice for each package is either buy the package and get all the items inside or don't buy the package and get nothing from the items that this package contains.
I want to know if this is possible to buy the packages so I get all what I need exactly as written in my grocery dotted list without having superfluous items that I don't need.
I will bring my mobile phone to the supermarket and once I can peek inside the packages I will run the application that will solve my problem.
I am programmer and I can program this application for my mobile phone with no problem but I want to use a polynomial time algorithm that solves my problem, because I am not going to wait hours, days, months and years until my phone will return me the answer.
But to know if there is polynomial algorithm or not I need to know first if my problem is NP-Hard or NP-Intermediate that polynomial solution is unknown.
Indeed my problem is decision, because I need to decide which packages I buy and which I don't buy and if I have a solution I can verify it in polynomial time, so my problem is indeed in $\mathbb{NP}$, but I don't know if it is in $\mathbb{P}$.
Do you know?
EDIT:
Mathematically my problem is show either:
$A\subset\mathbb{MULTISETS} \land B\in\mathbb{MULTISETS}\vdash\exists C: C \subseteq A \land \displaystyle \biguplus_{D \in C}D=B$
OR
$A\subset\mathbb{MULTISETS} \land B\in\mathbb{MULTISETS}\vdash\nexists C: C\subseteq A \land \displaystyle \biguplus_{D \in C}D=B$