From a gun weighing 9 tons, a projectile weighing 90 kg was shot in the horizontal direction

From a gun weighing 9 tons, a projectile weighing 90 kg was shot in the horizontal direction, which acquired a speed of 300 m / s relative to the ground. What speed did the gun acquire?

According to the law of conservation of momentum (the change in the momentum of the system is equal to zero), we have the following expression:
m * v = M * V, where v is the speed of the projectile, m is its mass, V is the speed of the gun, M is its mass.
Then (the mass of the tool must be expressed in kilograms):
V = m * v / M = 90 * 300/9000 = 3 m / s.