Given:
cA(n)=5n+50
cC(n)=4n+80
and cC(n)<cA(n)
so
4n+80 < 5n + 50
Subtract 4n from both sides
4n-4n + 80 < 5n-4n +50
80 < n + 50
Subtract 50 from both sides,
30 < n
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