Question 1069681
Maybe all that is expected is
{{{x}}}= mean score of the students failing the test
Since {{{6}}}= mean score represents a "weighted average" of
{{{"60%"=60/100=0.6}}} of the students with a mean score of {{{8}}}
and {{{"100%"-"60%"=40%=40/100=0.40}}} of the students with a mean score of {{{x}}} :
{{{0.4x+0.6*8=6}}}
{{{0.4x+4.8=6}}}
{{{0.4x=6-4.8}}}
{{{0.4x=1.2}}}
{{{x=1.2/0.4}}}
{{{highlight(x=3)}}}
 
A DIFFERENT EXPLANATION:
We are told that
{{{6}}}= mean score of the students who took the test
{{{8}}}= mean score of the students who passed the test 
and also that
{{{"60%"=60/100=0.60}}} is the fraction of students who passed the test,
meaning that unfortunately
{{{"100%"-"60%"=40%=40/100=0.40}}} is the fraction of students who failed the test.
Let us say that
{{{n}}}= total number of students taking the test,
and we want to find
{{{x}}}= mean score of the students failing the test
Then
{{{0.60n}}}= number of students passing the test
{{{0.40n}}}= number of students failing the test
{{{6*n}}}= sum of scores of all students taking the test
{{{8*(0.60n)}}}= sum of scores of students passing the test
{{{(0.40n)*x}}}= sum of scores of students passing the test
So.
{{{0.4nx+8*0.6n=6*n}}}
{{{0.4nx+4.8n=6n}}}
{{{(0.4x+4.8)n=6n}}}
{{{0.4x+4.8=6}}}
{{{0.4x=6-4.8}}}
{{{0.4x=1.2}}}
{{{x=1.2/0.4}}}
{{{highlight(x=3)}}}