A deer is running at constant speed. It watches a lion and increases its speed by $3m/s$ .If after watching the lion it travels $400m$ in $10s$.Find its velocity before it watches the lion.

Hint: The rate at which an object's position changes in relation to a frame of reference is called velocity, and it is a function of time. The term "velocity" refers to a description of an object's speed and direction of motion.

Complete step-by-step solution:
Speed is the rate at which an object moves along a path in terms of time, while velocity is the rate and direction of movement. In other words, while speed is a scalar value, velocity is a vector. Average velocity is determined by dividing the change in position \[\left( {\Delta r} \right)\] by the change in time \[\left( {\Delta t} \right)\] in its simplest form. Both terms are interchangeable because they mean the same thing, though when you're asked for a velocity, you'll usually be asked for both the speed and the direction.
Explanation:
Speed=distance/time
\[\begin{array}{*{20}{l}}
  { = 400/10} \\
  { = 40m/s}
\end{array}\]
\[ = 40m/s\]
:.speed after watching lion \[ = 40m/s\]
It increases speed 3m/s per second.
:.Total speed increased= \[3 \times 10 = 30m/s\]
Speed before watching lion = \[40 - 30 = 10m/s\]

Note:The coordinate system used to describe the position determines the sign of the velocity. A positive velocity indicates that the object is moving in the positive coordinate system direction, while a negative velocity indicates that the object is moving in the opposite direction.