I'll say that the unknown number is "x", then our expression is
[tex]12+x=13+7[/tex]And we want to find out the value of "x"
To solve it, we will first do the sum on the right side:
[tex]12+x=20[/tex]Now, we want to have "x" alone on the left side, then, let's subtract 12 on both sides
[tex]\begin{gathered} 12-12+x=20-12 \\ \\ x=20-12 \\ \\ x=8 \end{gathered}[/tex]Then the value of x is 8, let's test it:
[tex]\begin{gathered} 12+8=13+7 \\ \\ 20=20 \end{gathered}[/tex]Correct! 12 + 8 is 20. Then, the unknown number is 8
