The word “decreased” indicates the symbol “”.

The word “equals” indicates the symbol “=”

Let n resents the number.  

Rewrite the sentence into equation,

The equation is n 77 = 18.       

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

n 77 + 77 = 18 + 77       (Add 77 to each side )

  n =  18 + 77                       (Additive inverse property: 77 + 77 = 0)

   n = 59                                    (Add: 18 + 77 = 59

Check:

To check the solution, substitute n = 59  in original equation. 

                            (Subtract: )

Compare the values, the equation is true. The solution is n = 59.

The number is 59. \ \