An unknown is a quantity that is not specified.
There are 7 fish in the aquarium. Ali is going to add in some fish. The total number of fish becomes 10.
An unknown is some fish. 7 plus the unknown is 10.
Mathematical sentecnce:
\(7\space+\space\underline{\quad\quad}=10 \\\\\;\;\;\;\;\;\;\;\;\,\Uparrow \\\\\;\;\;\;\text{unknown}\)
Assess yourself by staged practices
