Question 890158
{{{(sum(x[i], i = 1 ,5))/5 = 11}}} by the given 

==>{{{sum(x[i], i=1,5) = 55}}} after cross-multiplication

==>{{{sum(x[i], i=1,5)+ x[6] = 55+ x[6]}}}

==>{{{(sum(x[i], i=1,5)+ x[6])/6 = (55+ x[6])/6 = 12}}} after dividing both sides by 6

==> {{{55 + x[6] = 72}}} after cross-multiplication.

Hence {{{x[6] = 17}}}