Theory:

Example:
Fill in the suitable values:
 
1. ____ \(+ 5 = 11\)
 
This can be done by trial and error method.
 
Let us substitute some values by our self and get the fit of it.
 
Let the unknown value is \(1\).
 
\(1+5 = 6\).
 
In the right side of the equation, we have \(11\). Thus, the value does not fit.
 
Let the unknown value is \(2\).
 
\(2+5 = 7\).
 
In the right side of the equation, we have \(11\). Thus, the value does not fit
 
Let the unknown value is \(3\).
 
\(3+5 = 8\).
 
In the right side of the equation, we have \(11\). Thus, the value does not fit.
 
Let the unknown value is \(4\).
 
\(4+5 = 9\).
 
In the right side of the equation, we have \(11\). Thus, the value does not fit.
 
Let the unknown value is \(5\).
 
\(5+5 = 10\).
 
In the right side of the equation, we have \(11\). Thus, the value does not fit.
 
Let the unknown value is \(6\).
 
\(6+5 = 11\).
 
In the right side of the equation, we have \(11\). 
 
Thus the value satisfies the given equation.
 
Therefore the unknown value must be \(6\).