Assertion: Rain is falling vertically with a certain speed. A boy holding an umbrella rides a bicycle in east to west direction and does not get wet.
Reason: The boy is holding his umbrella (at some angles) with the vertical towards the west.

Explanation

To analyze the question, we need to break down the assertion and the reason provided.

### Step 1: Understand the Assertion
The assertion states that rain is falling vertically at a certain speed, and a boy riding a bicycle from east to west does not get wet. This indicates that the boy's umbrella is effectively blocking the rain.

**Hint:** Consider the relative motion of the rain and the boy on the bicycle.

### Step 2: Analyze the Motion
Since the rain is falling vertically, it has a downward velocity component. The boy, riding from east to west, has a horizontal velocity component. The combination of these two motions creates a resultant direction of rain relative to the boy.

**Hint:** Think about how the boy's motion affects the perceived direction of the rain.

### Step 3: Determine the Direction of the Umbrella
For the boy to not get wet, the umbrella must be positioned in such a way that it intercepts the rain. If the boy is moving west, the rain will appear to come from a direction that is not purely vertical due to the boy's horizontal motion.

**Hint:** Visualize the path of the rain as seen from the boy's perspective.

### Step 4: Evaluate the Reason
The reason states that the boy is holding the umbrella at some angle with the vertical towards the west. However, for the boy to effectively block the rain, the umbrella should actually be tilted towards the east, not the west.

**Hint:** Compare the direction the umbrella needs to be tilted to the actual direction stated in the reason.

### Step 5: Conclusion
From the analysis:
- The assertion is true because the boy can indeed avoid getting wet if the umbrella is positioned correctly.
- The reason is false because it incorrectly states the direction in which the umbrella is tilted.

Thus, we conclude that the assertion is true, but the reason is incorrect.

**Final Answer:** Assertion is true; Reason is false.