Question 1202987
That just simple arithmetic.  What difficulty are you having?
n is 1, 2, 3, 4; what is each term?
Which index if the term is equal to 208?  Solve {{{n^2-n-2=208}}}.



The term being 208:

n^2-n=210
n(n-1)=210
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10*21, 20*10.5, 60*3.5
2*5*3*7
14*15
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Think 14 & 15.
n(n-1)=210=15*14
{{{highlight(n=15)}}}