Pls solve in steps,how did we get the formula for KE & PE

Pls solve in steps,how did we get the formula for KE & PE QI: 10 What are the speeds of the charged bodies A & B at infinite distance from its initial position, when both are released? Given the mass of both is 10 g. Charge on them are QA =100 1_IC, QB = 400 VIC.

What is their initial distance of separation? Questionb seems incomplete 

Potential Energy due to the charges kept at an initial distance say r

U = 1/4πε_o Q_AQ_B/r

Let v be their speeds.

At infinite distance the entire U will be converted to KE.

½mv² + ½mv² =  1/4πε_o Q_AQ_B/r

=> mv² =  9×10⁹ × 10^-4 × 4 × 10^-4/r

=> 10^-2v² =  360/r

=> v² =  360×10²/r

=> v = 60√10/√r

If initial separation is r = 1 m

=> v = 60√10 m/s