Question 5391
4,9,19,39,79

the thing is with these...spot a pattern of sorts. Now 4,9,19,39,79 means nothing, but what about 5, 10, 20, 50, 80? That is much more recognisable as a pattern...

n =    {{{1}}},     {{{2}}},      {{{3}}},      {{{4}}},       {{{5}}}
--> {{{5x1}}}, {{{   5x2}}}, {{{   5x4}}}, {{{   5x8}}}, {{{   5x16}}}
--> {{{5x2^0}}}, {{{5x2^1}}}, {{{5x2^2}}}, {{{5x2^3}}}, {{{5x2^4}}}


--> {{{5x2^(n-1)}}}


Now the original question is 1 less than each of these terms, so final answer is {{{5x2^(n-1) - 1}}}

jon.