Question 1022736
{{{t[n] = t[1]*r^(n-1)}}} ==> {{{t[n] = -60*(-1/2)^(n-1)}}}

==>  {{{t[20] = -60*(-1/2)^19 = 15/2^17 = 15/131072}}}, or 0.00011444 to eight decimal places.