Question 1041108
There are {{{X}}} questions on the test.
Of the first ten, you answer 9 correctly.
That leaves {{{X-10}}} questions left on the test since,
{{{10+X-10=X}}}
So then scoring (3/10) of the remaining gives you,
{{{(3/10)(X-10)}}} additional correct answers. 
So the total number of correct answers is,
{{{9+(3/10)(X-10)}}}
and that equals 50% of all of the questions, X.
{{{9+(3/10)(X-10)=(50/100)X}}}
Multiply both sides by 10.
{{{90+3(X-10)=5X}}}
{{{90+3X-30=5X}}}
{{{2X=60}}}
{{{X=30}}}