addition and subtraction facts


Senior Member
Southern Italy; Italian
Hello everybody!
I'm translating a Primary School Program and I'm stuck on this sentence:

Know by heart all addition and subtraction facts for each number to 20, and pairs of multiples of 100 with a total of 1000.

It deals with mathematics and I don't understand what "addiction and subtractions facts" means.
It's something dealing with "addizione" and "sottrazione"...but what exactly?

My attempt is: Conoscere a memoria tutti i FACT dele addizioni e delle sottrazioni, per ciascun numero fino a venti e per le coppie di multipli di 100 con un totale di 1000"
...but it is nonsense to me...
Can anybody of you help me??
Thanks a lot!!
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