 We've already spoken about the difference between instantaneous and average properties and so if we look at a very small piece of time of time delta t then a particle moves a distance delta x and our instantaneous velocity is going to be just that delta x over delta t. So while you might use the formula that the velocity is a change in position over a change in time, it comes up in a lot of different contexts. You have to make sure whether you're talking about things like average velocity or the instantaneous velocity and of course most commonly the velocity is changing over time or there might be more than one thing so there's more than one velocity. So whenever you're dealing with a real problem make sure that you draw a diagram and make sure that you write this particular symbols you're talking about and you're very clear and use words to describe what you're talking about when you talk about these symbols.