**Force in CGS system is 20 N. Its value in SI unit will be:**

To convert a force measured in the CGS system to the SI system, we need to understand the relationship between the two systems.

**1. Understanding the CGS System:**
The CGS (centimeter-gram-second) system is one of the metric systems of units. In this system, the base units are centimeters for length, grams for mass, and seconds for time. The unit of force in the CGS system is dyne, which is defined as the force required to accelerate a mass of 1 gram by 1 centimeter per second squared (1 dyne = 1 g * cm/s^2).

**2. Understanding the SI System:**
The SI (International System of Units) is the modern form of the metric system used worldwide. In this system, the base units are meters for length, kilograms for mass, and seconds for time. The unit of force in the SI system is newton, which is defined as the force required to accelerate a mass of 1 kilogram by 1 meter per second squared (1 N = 1 kg * m/s^2).

**3. Conversion Formula:**
To convert a force from the CGS system to the SI system, we can use the following conversion formula:

Force_SI = Force_CGS * (1 dyne / 1 N)

**4. Calculation:**
Given that the force in the CGS system is 20 N, we can substitute this value into the conversion formula:

Force_SI = 20 N * (1 dyne / 1 N)

**5. Simplification:**
We know that 1 N is equal to 10^5 dynes (1 N = 10^5 dyn). Therefore, we can rewrite the conversion factor as:

1 dyne = 1 N / (10^5)

Substituting this into the equation:

Force_SI = 20 N * (1 N / (10^5 N))

**6. Final Calculation:**
Simplifying the equation further:

Force_SI = 20 N * (1 / 10^5)

Force_SI = 20 * 10^(-5) N

Force_SI = 2 * 10^(-4) N

Therefore, the value of the force in the SI system is 2 * 10^(-4) N.

In summary, the force of 20 N in the CGS system is equivalent to 2 * 10^(-4) N in the SI system.