Introduction:
When a bullet is fired from a pistol, the pistol experiences a recoil due to the conservation of momentum. According to the law of conservation of momentum, the total momentum before the bullet is fired is equal to the total momentum after the bullet is fired.

Given:
Mass of the bullet (m₁) = 20 g = 0.02 kg
Velocity of the bullet (v₁) = 150 m/s
Mass of the pistol (m₂) = 2 kg
Recoil velocity of the pistol (v₂) = ?

Calculating the momentum of the bullet:
The momentum of an object is given by the product of its mass and velocity.
Momentum of the bullet (p₁) = mass of the bullet (m₁) * velocity of the bullet (v₁)
p₁ = m₁ * v₁
p₁ = 0.02 kg * 150 m/s
p₁ = 3 kg·m/s

Calculating the initial momentum of the pistol:
The initial momentum of the pistol is equal to the negative of the momentum of the bullet, as the total momentum before firing is zero.
Initial momentum of the pistol (p₂) = -p₁
p₂ = -3 kg·m/s

Calculating the final momentum of the pistol:
The final momentum of the pistol is equal to the product of its mass and recoil velocity.
Final momentum of the pistol (p₃) = mass of the pistol (m₂) * recoil velocity of the pistol (v₂)
p₃ = m₂ * v₂

Applying the law of conservation of momentum:
According to the law of conservation of momentum, the total momentum before the bullet is fired is equal to the total momentum after the bullet is fired.
Initial momentum of the system (p₁ + p₂) = Final momentum of the system (p₃)
-3 kg·m/s + p₁ = m₂ * v₂

Calculating the recoil velocity of the pistol:
Rearranging the equation, we can solve for the recoil velocity of the pistol.
v₂ = (p₁ + 3 kg·m/s) / m₂
v₂ = (3 kg·m/s + 3 kg·m/s) / 2 kg
v₂ = 3 m/s

Conclusion:
The recoil velocity of the pistol is 3 m/s in the opposite direction of the bullet's velocity. When the bullet is fired with a velocity of 150 m/s, the pistol experiences a recoil velocity of 3 m/s in the opposite direction to conserve momentum.