**Stopping a Bullet**

To determine the number of planks required to completely stop a bullet, we need to understand how the bullet's velocity decreases as it passes through each plank.

**Bullet Velocity Reduction**

Given that the bullet loses 1/50 of its velocity when passing through one plank, we can calculate the remaining velocity after each plank as follows:

- After the first plank: The bullet retains 49/50 (or 98%) of its initial velocity.
- After the second plank: The bullet retains (49/50) * (49/50) = (49/50)^2 (or approximately 96.04%) of its initial velocity.
- After the third plank: The bullet retains (49/50)^3 (or approximately 94.12%) of its initial velocity.

We can notice that the bullet's velocity decreases exponentially with each passing plank.

**Stopping the Bullet Completely**

To stop the bullet completely, we need to determine the point at which the bullet's velocity becomes negligible. We can consider a velocity of less than 1% of the initial velocity as negligible.

Let's assume the initial velocity of the bullet is V. We can set up an equation to find the number of planks required to reduce the velocity to less than 1% of V:

(49/50)^n * V < />

Simplifying the equation, we get:

(49/50)^n < />

Taking the logarithm of both sides, we have:

n * log(49/50) < />

Solving for n, we find:

n > log(0.01) / log(49/50)

Using a calculator, we find that n > 137.51.

Since the number of planks must be a whole number, we round up to the nearest integer. Therefore, we need at least 138 planks to completely stop the bullet.

**Conclusion**

To completely stop a bullet that loses 1/50 of its velocity when passing through one plank, we require at least 138 planks.