Define a variable and write an expression for the phrase. the quotient of 6 times a number and 16

A variable is (usually) a letter from the alphabet that we use in place of an unknown number. We use this to place it into equations and solve for the equation (meaning you solve the unknown number inside the equation). Other variables that aren't letters from the alphabet are used in other classes such as Physics, where Greek letters from the Greek alphabet are used.

X=(6Y)/16
X="The Quotient"
Y="Number"
This cannot be solved BECAUSE you have 2 unknown variables, X and Y (Could've been any other letters but X, Y, and Z are most oftenly used in math..)

Because you have only one equation using two unknown numbers, X and Y, you cannot solve this, however, you have solved the problem of "write an expression for the phrase" correctly, as this is an expression.