Suppose we want to choose 2 letters, without replacement, from the 4 letters A, B, C, and D. How many ways can this be done, if the order of the choices matters?
How many ways can this be done, if the order of the choices does not matter?

The tiles below are pieces of a normal ecg tracing representing a little less than two heartbeats at a resting rate of 80 bpm. arrange the tiles in their correct order. rank from first to last. do not overlap any tiles.

What could explain what happened when the time was equal to 120 minutes

Ascientist wants to use a model to present the results of his detailed scientific investigation. why would a model be useful? a )because the model makes the concepts easier to understand b) because the model is easy to put together and to use c) because the model prevents other scientists from asking questions d) because the model requires the audience to pay full attention to it i don't know what to choose because between a and d a is much more likely than d because it only wants to get the attention of the audience. i'm confuse.