Let n represent the number.

The word “equals” can be represented as “=” symbol.

The word “decreased” indicates subtraction and the symbol is “” .

The word “twice the number” can be represented as “2n”

Rewrite the sentence into expression as \ \

The expression is 12 2n = 34. \ \

According to subtraction property of equality: If a = b then a – c = b – c.

12 2n 12 = 34 12    (Subtract 12 from both sides)

12 12 2n = 34 12    (Commutative property: a + b = b + a)

2n = 34 12                     (Additive inverse property:   12 12 = 0)

2n = 46                              (Add: 34 12 = 46)

                           (Divide both sides by -2)

                                   (Cancel common factors)

n = 23                                         (Divide: )

The number is 23.