Acceleration of skier on slope

This might be an extremely easy question and I'm just over-thinking it, but could someone give me a hand with some grade 10 physics?

1. A downhill skier has an initial speed of 2.5 m/s. She accelerates up to a speed of 20 m/s in 3.8 s.

a) Calculate the average acceleration of the skier.

Okay, so I know that acceleration equals velocity over time, but I'm slightly confused. So I know how to calculate the acceleration of 20 m/s in 3.8 s. Which is ... (about) 5.26. But, I get stuck here.

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hage567
Homework Helper
Why are you confused? Explain.

I don't know how to find the average acceleration.

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wait, is that all I have to do?

hage567
Homework Helper
No, your answer isn't correct. Average acceleration is the change in velocity over a given time interval. So what's the velocity at the beginning of your time interval, and what is it at the end?

Do you recognize this: $$a_{ave} = \Delta v / \Delta t$$

$$a_{ave} = \frac{\Delta v}{\Delta t} = \frac {v_{final} - v_{initial} }{t_{final} - t_{initial} }$$

it is definition

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