Answer:
A. The initial velocity of the bullet is 
B. Mechanical energy of the system before and after collision: 451.80 J, 0.9018 J
C. Percentage of K.E lost to heat is = 99.8 %
Explanation:
From conservation of linear momentum,

let the mass of the block be m1 and velocity = v1
let the mass of the bullet be m2 and velocity = v2
Let the final velocity of the system be v.
A. Plugging our parameters into the equation, we have:
![[(5 \times 0) +(0.01\times v_{2})]= 5.01 \times 0.6](https://tex.z-dn.net/?f=%5B%285%20%5Ctimes%200%29%20%2B%280.01%5Ctimes%20v_%7B2%7D%29%5D%3D%205.01%20%5Ctimes%200.6)

Hence, the initial velocity of the bullet is 
B. The mechanical energies of the system exist in form of kinetic energy.
I. Kinetic energy of the system before collision:

II. Kinetic energy after collision:

C. Change in Mechanical Energy = 

Percentage of K.E lost to heat is = 99.8 %