It adds up...

[kachibi]

I looked up "add up" in the Cambridge Dictionary and I saw these two examples:

If you buy three packets, it adds up to a total of $18.
Cinema tickets, drinks, snacks - it all adds up to a lot a money.

Can someone tell me why IT is used but not THEY in both examples? Shouldn't THEY be used to refer to the "three packets" or "tickets, drinks, snacks"?

 

[dojibear]

The sentences are taking about the price of the three packets, not the packets.
The prices of the three packets adds up to a total of $18.
The prices of the cinema tickets, drinks and snacks adds up to a lot of money.

 

[kachibi]

Do you mean this "it" refers to the TOTAL SUM of all prices so that "it" is used?

If yes, does that mean, for most other situations, when we use "add up...to a value" to talk about the values of more than 1 item, be it money, weight, height, amount, the subject MUST BE SINGULAR because the TOTAL SUM instead of the separate values is considered?

Just like: Peter, Mary and Tom- it all adds up to 250kg.