Question 460755
{{{a[1] = 8}}} and d = 7 ==>  {{{a[14] = a[1] + (n-1)d = 8 + 13*7 = 99}}}

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{{{a[1] = 6}}} and {{{a[5] = 18}}} and n = 5.

==> {{{a[5] = a[1] + 4*d = 6 + 4d = 18}}} ==> 4d = 12 ==> d = 3.

The arithmetic means are 9, 12, and 15.