One reason would be if you did not know the answer so you could do
4-5=x
another reason would be if the problem gave you the answer but not one of the things to get the answer
4-x=-1
not sure if that is what you want but I hope this helped
Answer:
x = 10
Step-by-step explanation:
8 + 12 = 2x
8 + 12 = 20
20 = 2x
---- -----
2 2
10 = x
x = 10
Given:
The system of equations is


To find:
The missing value for which the given system of equations have infinitely many solutions.
Solution:
Let the missing value be k.
We have,


Taking all the terms on the left side, the given equations can be rewritten as


The system of equations
and
have infinitely many solutions if

We have,


Now,



On cross multiplication, we get


Therefore, the missing value is -10.
I think this is more of an opinion question. What someone could solve mentally may not be the same as someone else's.If it were my worksheet, I'd circle all of them but it may be different for you.
At your level, I'd probably circle:
54 - 10
93 - 20
and 39 - 2