help please

The best way to solve this problem is to listen to some music while you read the textbook.

The usage of "common difference" implies it is an arithmetic sequence.
We can use the formula for an arithmetic sequence: Sn=(n(a1+an))/2, or Sn=(n(a1+d(n-1)))/2

a1=4, an=199, sn=4060
As an=199 and a1=4,
8120=n*(199+4)
n=40
199=d(n-1)=39d
d=5

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