The equation is .          

According to addition property of equality; if a = b, then a + c = b + c.

                (Add 5 to each sides)

                           (Additive inverse property: )

                                   (Add: 15 + 5 = 20)

According to division property of equality:

For any number a, b and c, If a = b then .

                               (Divide both sides by 4)

                                   (Cancel common terms)

a = 5                                        (Divide: )

Check:

To check the solution, substitute a = 5 in original equation.     

                                        (Multiply: )

                                    (Subtract: )

Compare the values, the equation is true.

The value of a = 5.