I found this question on some Japanese TVshow and I could not figured out.
Find the radius of this black holes. when G= 6.67*10^-11 m^3.kg^-1.s^-2 , c = 3*10^8 , black hole's mass = 4.20*10^40 kg
Can u find the answer with these information.

 

They probably mean the radius of the event horizon, which is simply the radius at which the escape velocity is the speed of light.

 

How do you solve this problem?

What problem?

Would it not be better to state the problem in the title in the first place?

Such as: Find the radius of this black hole

 

The Swarztschild radius is calculated thusly:

Rs = 2GM/(C^2)