To find: The time taken by the Glacier. Given: The lenght of Glacier is 31680 ft, and his speed displaystyle={31680} ft per year.

Concept used:
Consider the following steps to solve one variable linear inequality:
If an equation contains fractions or decimals, multiply both sides by the LCD to clear the equation of fractions or decimals.
Use the distributive property to remove parentheses if they are present.
Simplify each side of the inequality by combining like terms.
Get all variable terms on one side and all numbers on the other side by using the addition property of inequality.
Get the variable alone by using the multiplication property of equality.
Check the solution by plugging it into the original equation.
Translate the problem into an equation as,
${\displaystyle 400x=31680}$
${\displaystyle \frac{400x}{400}=\frac{31680}{400}}$
${\displaystyle x=79.2}$
Thus, the time taken by the Glacier is 79.2 years.