#### Answer

$$\frac{x}{5}-2 = 18 $$

#### Work Step by Step

First identify the task, which is to express "the quotient of a number and five, decreased by two, is 18" as an equation. We are told to assume the number is "x". A method to solving this problem is to divide the given statement into sub parts and then convert these into an equation.
The quotient of a number and 5 can be expressed as "$\frac{x}{5}$".
The quotient is decreased by two can be expressed as "$\frac{x}{5} -2 $".
The solution is given as 18 can be written "$=18$".
By putting these together, the answer is $\frac{x}{5}-2 = 18 $.