Today Lily is 15 years old and Paul is 42 years old. In how many years the age of Paul will be double that of Lily?

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This type of exercise can be solved using the function : equation_solver

An equation is an equality that involves one or more variables, solving an equation in a set is to find the value or values of the variables in that set that verify the equation,
these are the solutions of the equation. The variables are often referred to as the unknown,
when the equation has only one unknown, it is often referred to as x.

When we have to solve several equations, with several variables, we speak of a
system of equations.

There are methods and formulas for solving certain types of equations such as first degree equations (linear equation),
second degree equations (quadratic equation), or product equations.

Solve an equation with one unknown of the first degree

Solving an equation with one unknown x in R, it is to determine the set of real numbers x verifying the said equation.
This set is called the set of solutions of the equation.

When we add or subtract the same real to both members of an equation, we obtain a new equation which has the same solutions as the previous one.

When we multiply or divide the two members of an equation by a non-zero real, we obtain a new equation that has the same solutions as the previous one.