turning point

Hi,

I am not sure how to find the turning point to this graph. I have seen an answer to this and the turning point is at x=15.
How do I get the turning points for such a graph?

^ is power to

f(x)=x^3 (/160) * (20-x)
jagguyAsked:
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ozoConnect With a Mentor Commented:
The derivative would be 0 at turning points
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