### 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 $$6$$.