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 angle) with the vertical towards west. 

Rain is falling vertically with a certain speed. The boy rides a bicycle in east to west direction. So with respect to the boy rain will appear to have a horizontal velocity that is equal in magnitude of the boy but in opposite direction.

So the rain appears to have a horizontal velocity of west to east.

In-order to avoid getting wet the boy holds the umbrella at some angle with vertical towards west.