Let's denote the man's current age as $M$ and his son's current age as $S$.

According to the problem, we have two pieces of information:

1. The man's age is four times that of his son: $M = 4S$

2. In 20 years, the man will be twice as old as his son: $M + 20 = 2(S + 20)$

We can solve these equations step-by-step.

First, substitute the first equation into the second equation: $4S + 20 = 2(S + 20)$

Now, expand and simplify: $4S + 20 = 2S + 40$

Subtract $2S$ from both sides: $2S + 20 = 40$

Subtract 20 from both sides: $2S = 20$

Divide both sides by 2: $S = 10$

Now that we have the son's age, substitute $S = 10$ back into the first equation to find the man's age: $M = 4S$ $M = 4 \times 10$ $M = 40$

So, the man's current age is $40$ years old.

Would you like any more details or have any questions about this solution? Here are five more questions you might find interesting:

1. How can you find the roots of a quadratic equation?
2. What is the derivative of $x^3 + 3x^2 - 4x + 7$?
3. How do you solve a system of linear equations using the matrix method?
4. What is the integral of $\sin(x) \cos(x)$?
5. How do you calculate the area of a triangle given its three vertices?

Tip: When solving word problems, it's helpful to clearly define variables and write down all given relationships as equations.